URBANDALE, Iowa—One scorching, airless early evening in the middle of August, on the outskirts of Des Moines here at a place called Living History Farms, Joe Biden stood in front of an old yellow barn and talked to a couple hundred people about the past.

“I think that, uh, the behavior of this administration has awakened, uh, a whole new generation to get engaged in ways that they may not have gotten before,” Biden said, referring to President Donald Trump and the current tumult. “Just like in my generation, when I got out of school that, uh, when Bobby Kennedy and Dr. King had been assassinated in the ‘70s, uh, late seven—when I got engaged, um, you know, up to that time, remember the, none of you women will know this, but a couple men may remember, that was a time in the early, late ‘60s, and the early ‘60s and ‘60s, where it was drop out and go to Haight-Ashbury, don’t get engaged, don’t trust anybody over 30. I mean, for real. What happened to them, by the, by the early ‘70s, the late ‘60s, there was a whole generation that said, ‘Enough.’ The war in Vietnam was under way, and it was—a lot of you served in that war—and, uh, we were fighting like the devil to make sure that there was something dealing with cleaning up the environment, which was only beginning. We were in a position where the women’s movement was just beginning to move. We should have, by now, long before, passed the ERA amendment, but that was another issue …”

Sticky-squeezed into plastic chairs, the torpid crowd used handed-out campaign paraphernalia to fan their sweaty faces. But at this mention of the equal rights amendment, somebody started to clap, and others followed suit, and the smattering of applause felt like an act of mercy—giving the characteristically discursive Biden a chance to reset and everybody else the opportunity to take a breath and maybe not think too hard about the fact that the former vice president had bungled by a decade the dates of two of the most jarring and consequential killings in modern American history.

After Biden finished talking, he spent the better part of an hour mingling with the many who wanted to stick around for selfies. Clad in brown loafers, trim tan slacks and a snug navy-blue polo shirt that accentuated the lean musculature of his tan upper arms, he worked the rope line with vigor and veteran aplomb, shaking hands, kissing cheeks and smiling to flash his teeth that are an even brighter shade than his wispy white hair.

A young staffer with a clipboard asked a man within my earshot if he wanted to sign up to commit to caucus for Biden.

“Already filled it out,” the man responded. “I think I did that in ’87.” The first time Biden ran for president—32 years ago.

This event, like every Biden event, couldn’t help but highlight one of the defining realities of his 2020 candidacy: Next month, he turns 77 years old. His age is the subtext, and increasingly the text, too, of not only his bid but the Democratic Party’s primary as a whole. Even as fading poll numbers loosen his status as the favorite and the mounting impeachment fervor over Ukraine threatens to exact a collateral toll, Biden’s age remains an overarching issue.

It’s an issue because of the simple math: Only three presidents have served in their 70s—Trump, Ronald Reagan and Dwight Eisenhower—and no president ever has finished a term at the age Biden would begin his. It’s an issue because of things Biden has said and done—suggesting, for instance, he thought he was in Vermont when he actually was in New Hampshire, dropping that wince-worthy phrase that his “time is up” in the first debate and oddly invoking a record player in the last debate. His lifelong habit of flubs, gaffes and often garbled speaking now can seem less like “Joe being Joe” and more like an ominous indicator of a creeping loss of mental acuity.

It’s an issue because Biden himself has tried in sometimes awkward ways to keep it from being one, inviting a heckler to run with him during a parade and challenging a reporter to a wrestling match. And it’s an issue because opponents, from Trump (“Sleepy Joe”) to those in his own party trying to knock him off, have made it an issue—from Eric Swalwell saying it was time to “pass the torch” to Tim Ryan saying he’s “declining” to Julian Castro (dubiously) accusing him of “forgetting” things to Cory Booker dishing out readymade Republican attack ad fodder by bluntly declaring on CNN that “there’s a lot of people who are concerned about Joe Biden’s ability to carry the ball all the way across the end line without fumbling” and “there are definitely moments where you listen to Joe Biden and you just wonder.” Even the famously gracious Jimmy Carter, who just turned 95, delivered a candid if unhelpful assessment last month when he said he didn’t believe he could have handled “the duties that I experienced when I was president” if he had been 80.

This isn’t just about Biden’s age—it’s about ours, and the tension between a vast cohort of Baby Boomers who have trained themselves to believe they’re only as old as they feel and a couple of impatient generations lined up behind them, wondering when they’re going to get a chance to take over. And yet it’s about far more than simply a number next to a name. Our sense of who is old in this primary has become entwined with our appetite for bold and new ideas. All three of the top-polling Democrats, after all, are in their 70s, but it’s Biden, the centrist who advocates for a return to a pre-Trump time, who is getting dinged the most for his advanced age—not Elizabeth Warren, who wants “big, structural change” and turned 70 in June. Up until this week when he had to have two heart stents implanted, neither was Bernie Sanders, who continues to call for his “revolution” and is in fact the oldest of the lot.

But there is an entire cohort of Biden supporters for whom his age—actual and perceived—is the very thing that recommends him. After the tumult of the Trump years, these voters crave the experience and order and stability Biden promises. For them, Biden is the beneficiary of shifting social and cultural notions that make it harder to pinpoint what it actually means to be old. Federal law protects workers from age discrimination starting at 40. People can join AARP at 50. They’re usually eligible for Medicare at 65 and Social Security at 66. Scientifically, though, a half-dozen aging experts I talked to for this story told me, there’s such vast variability in how people age that it’s ill-advised and even irresponsible to try to draw conclusions about an individual based on a date of birth. “There are people at 80 who perform better than 20-year-olds,” said Christopher Van Dyck of Yale University, “even on these cognitive speed, memory-type tasks.” Furthermore, beyond decades of a healthy diet and sufficient exercise, a significant, intangible, practically mysterious part of the nature of anybody’s aging, said Tracy Chippendale of New York University, is just … luck. Genes. Joe Biden’s father died at 86. His mother died at 92. People, said Denise Park of the University of Texas at Dallas, have to make a determination “based on the behavior that they observe.”

That’s essentially what Biden’s repeatedly asked voters to do.

“Watch me,” he said in June. “Just watch me.”

Over the last six weeks, in three of the most important states in the primary process, I watched him—in South Carolina, in New Hampshire, and here in first-to-caucus Iowa.

In Prole, in an oversized lakeside gazebo, I watched him talk about the 1951 Chevy he drove in high school as he sweat through his shirt. “I think we want to have him in the shade, guys,” said an aide to the cluster of reporters who had gathered around him to ask questions. “I don’t want him standing in the sun again,” said another. Biden was asked if he had thought about pledging to just one term to allay the concerns about his age. “No,” he said.

In Newton, blessedly inside at a renovated brick building that used to be a Maytag plant, I watched him tell a crowd about how he “got elected as a 29-year-old kid” and how he’s “met every single major foreign leader that has existed over the last 35 years.” Attendees I talked to, many of whom had white or gray hair themselves, professed to be impressed. “For a guy that’s as old as he is,” Tom Spidle, 64, teeing up what came out as a backhanded compliment, told me, “he looks incredible.” Chuck Walraven, 66, of Oskaloosa agreed. “To stand up there and talk as much as he did?” he said. “I mean, he’s not gonna fall over dead any more than you are, you know what I’m saying? I mean, you could die tomorrow—right now—talking to me! He could live to be a hundred.” Bruce Hoffmeier is the same age as Biden. He wore hearing aids and leaned on a cane. “He’s not too old,” Hoffmeier said on his way out. “He looks good.”

And at Living History Farms, I watched Biden keep working that rope line before turning to catch up with the man I’d overheard tell the young staffer that he’d committed to caucus better than three decades back.

Tom Rial of Des Moines was enrolled at the University of Northern Iowa, where he was the president of the student group for Biden. He supported him as well when he ran for the second time in 2007. He’s supporting him again now.

“He’s just got stature,” Rial, 52, told me. “Stature, maturity and experience.”

“The good sides of age,” I offered.

“He’s been around,” Rial granted.

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We started freaking out about the age of our presidents because of something that happened exactly 100 years ago this week.

On October 2, 1919, Woodrow Wilson collapsed in a bathroom at the White House, felled by a stroke that paralyzed his left side and rendered him incapacitated for the last nearly year and a half of his term. He was, the White House head usher would recall, “helpless.” Wilson, 63 when he was stricken, didn’t hold a cabinet meeting for more than seven months. His aides and his wife banded to do the work of the administration while attempting, too, to obscure the extent of his infirmity. But senators and staff who visited him saw “an emaciated old man” and “a very old man” who “acted like one.” It was, in the assessment of one of Wilson’s biographers, “the worst crisis of presidential disability in American history.”

Wilson was on the mind of Vice President Richard Nixon more than 30 years later, when President Eisenhower had a heart attack in September of 1955. Eisenhower recovered, enough to run for and win re-election the following fall, standing in open cars, waving to clamoring crowds, convincing the public that “suggestions that he was near death’s door were visibly untrue.” His ailing health in his second term, though, helped stoke the rise of John F. Kennedy, still the youngest president ever elected.

But the age-related episode the historians and operatives I talked to for this story brought up first unspooled over the course of two weeks in October of 1984.

Reagan, the oldest president ever elected until Trump, in the first of the two general election debates against Democratic nominee Walter Mondale that year appeared alarmingly ill-equipped and scatterbrained. He looked, according to Peter Goldman and Tony Fuller of Newsweek in their book about the campaign, “to be searching his mind and coming up empty.” He looked “damaged.” He “looked old,” they wrote. Reagan was 73, and even the typically friendly Wall Street Journal published a front-page article asking if that was too old, quoting a professor of medicine who said Reagan was shifting from “young-old” to “old-old.” A pro-Reagan psychologist added, “I’d be concerned to put him into a corporate presidency. I’d be all the more concerned to put him into the U.S. presidency.” All three networks did stories on it as well, airing an especially discomfiting portion of his performance: “The system is still where it was in regard to the uh, the uh, the uh, the uh …”

In the second debate, though, Reagan the ex-actor uncorked what even Mondale would acknowledge as “one of the great lines in the history of presidential debates.” Asked by one of the moderators about his age and his ability “to function,” Reagan responded, “I will not make age an issue of this campaign. I am not going to exploit for political purposes my opponent’s youth and inexperience.” The audience laughed. Mondale laughed. But he knew he was done.

“The joke,” Mondale wrote in his memoir, “completely disarmed people’s doubts about his age and his capacities and allowed them to think, ‘He’s okay.’ They wanted Reagan to be okay and now they could believe it.”

“People didn’t want him to fail,” University of Texas at Austin presidential historian H.W. Brands told me in an email. “His ability to joke about the issue told them it couldn’t be too serious.”

“Everybody laughed,” said longtime Democratic strategist Joe Trippi, who led Mondale’s efforts in Iowa that cycle, “and regardless of what the facts were about whether age was impacting him or not, it was over—that question was gone.”

Age matters. Because health matters. “Eventually, something like what happened to Woodrow Wilson is going to happen to us again,” Nixon biographer John Aloysius Farrell said in an interview. “I’ve seen almost everything—an assassination, two impeachments—in all my years as a watcher of politics. But I’ve never seen a president die of natural causes in office—and, you know, we’re due.” But voters don’t think like actuaries. Especially not right now. Democrats just want to beat Trump, and polls still say they think Biden’s the best bet.

Also, the challenge for Mondale in the scenario in ’84 looks in retrospect like a version of what Biden’s opponents are dealing with now: He had to try to diminish Reagan while simultaneously showing something along the lines of deference. The late Pat Caddell, the madcap mastermind who that year was an adviser to Mondale, understood how delicate a task it was and how hard it would be. Mondale, he reasoned in a strategy memo, had to “convince voters that Reagan has ‘lost it’ and that he ought to be retired. … In short, we want to have the American electorate emulate the British electorate in 1945 when they turned on Winston Churchill.” Still, the hit had to be deft. He all but likened it to putting a family elder in a nursing home—“sort of embracing a grandfather,” Caddell wrote, “and gently pushing him aside.”

Mondale couldn’t do it, and none of the younger 2020 candidates have managed it yet with Biden, either.

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Last month in New Hampshire, in Laconia, on the top floor of “the oldest unaltered brick textile mill in the U.S.,” according to the adjacent historical marker, I watched Biden tell people the salary for a senator back when he was first elected was $42,000 a year, briefly mix up Reagan with Nixon, and refer to Charlottesville as Charlotte before correcting himself two and a half minutes later.

“I found the vice president inspiring,” David Huot told me outside. Almost eight months older than Biden, he’s still a state representative. “Why the hell should I say he’s old?” Huot said. “I’m old.”

More often than not, though, I didn’t even have to ask about age.

After Huot, for instance, I met Dr. Paul Sapir, 91, and his wife, Sylvia, 76, Rhode Island residents with a nearby summer home, and asked them open-endedly what they thought of Biden’s performance.

“I was very impressed,” she said. “You know, I’ve been very worried about him in terms of his gaffes, but he seemed to know facts and figures, and they seemed to come to him easily.” Her husband, a retired psychiatrist, joined in. “It was quite reassuring about him and his intactness and his coherence,” he said. “He seems pretty intact.”

Intact. Sapir sounded like an archeologist describing a papyrus scroll.

That evening, in nippy New Castle over on the Atlantic coast, a man stood at the rear of the crowd in an oceanfront park and poked fun with a held-high sign welcoming Biden back to Vermont. In the question-and-answer session after his talk, Biden called a 24-year-old woman who asked about climate change “kiddo.” She said she found it “patronizing.”

The next morning, at the state Democratic Party convention at the sports arena in downtown Manchester, Biden was the first of all the presidential candidates to take the stage. The crowd was still sparse. He stifled coughs. By far the most memorable part of his speech was when he called Donald Trump “Donald Hump.”

People snickered.

Biden adlibbed. “Freudian slip,” he said.

The malaprop made for a droplet of entertainment. But the more meaningful development of the day, it seemed, happened not out in the open hall but rather in the bowels of the venue, in a stuffy, fluorescent-lit room, in which the roster of Oval Office aspirants could come talk to the assembled throng of reporters. Biden opted not to show up. Virtually everybody else did, though, and what unfolded in his absence was a collective crescendo of talk about the age issue—a preview of sorts of what would come the following week during and after the Houston debate. Ryan, 46, pointed to “a lack of clarity” in the way Biden speaks. Buttigieg, 37, said Democrats traditionally win the presidency with new, fresh faces, not “established and Washington-tenured figures.” Booker, 50, managed to turn a question about climate change into a chance to call attention to his relative youth. “I’ve seen military reports about what’s going to happen in the next 25 years—when, as another one of my colleagues says, I will still be younger than the president of the United States.” And Castro, days before he would unleash toward Biden his “forgetting” fusillade, was explicit. “I believe that we need a new generation of leadership,” he said, citing party presidential antecedents Kennedy, Carter, Clinton and Obama. “I believe people are looking for that again.”

Biden, however, remains the candidate to beat—in spite of the softening of his standing in some polls.

Older political hands think they know why.

“It’s perfectly all right to call for a new generation of leadership,” longtime Democratic strategist Bob Shrum told me. But he warned of “the danger of trying to go after Biden.”

“There is,” former top Obama strategist David Axelrod emailed, “a lot of affection and respect among Democrats for Biden. He is popular. Gratuitous attacks on him are not well received. And in an environment in which defeating Trump is the urgent concern of many Democrats, Biden seems like the safest, least risky choice.”

It’s why Joe Trippi looks at the 2020 primary to this point and finds himself thinking back to the 2010 California gubernatorial campaign. Jerry Brown was pitted against Meg Whitman. Trippi worked for Brown. Whitman was 54 and was coming off a decade of having been the CEO of eBay. Brown, on the other hand, had been around for just about forever, and had done just about everything—governor already before, mayor of Oakland, attorney general, three times a presidential candidate. He was 72. But the political terrain in the state coming out of eight years with Governor Arnold Schwarzenegger and in the wake of the Great Recession was jangled. Trippi was “blown away” by what he learned from voters in focus groups.

“We would give them this really stellar bio of Meg Whitman, and they all went, ‘No way.’ And you’re like, ‘What?’ And it was like, ‘No way—we tried somebody from outside government who didn’t know what the hell they were doing. … It was just too chaotic. We don’t want anybody new. We don’t want anybody from outside. Just somebody who knows how to turn the lights on in Sacramento. … Can’t you just give me an old shoe? I just want an old shoe.’”

In the case of that race, Trippi explained, they won by looking not forward so much as back.

Maybe that’s what’s at work right now.

“With age comes experience,” he said, “and with a chaotic president, the experience may count more than the age.”

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It may. But that's the exception to the rule. Usually it’s the other way around.

In 1972, for example, Cale Boggs was a two-term U.S. senator from Delaware. He was 63. Reporters in their coverage of the campaign described his “thinning hair” and said he sometimes got “tongue-tied.”

His challenger, meanwhile, a one-term county councilman, was 29. Reporters called Joe Biden “boyishly slim” and noted that he often talked about “how the old guard has bungled things.”

Biden ran newspaper ads yoking the incumbent to imagery evocative of bygone days. “Cale Boggs’ generation dreamed of conquering polio,” one said. “In 1950 Cale Boggs hoped to make Americans safe from Stalin,” a second said. Another mentioned “the 1948 poll tax.” The ads emphasized the need for “new thinking” and “new solutions.”

Boggs was “a helluva nice guy” but “was just not a fighter,” Biden said. He wondered if he had “lost that twinkle in his eyes.”

The steely sketching of this contrast was “pretty much the essence” of the Biden campaign’s strategy, according to one of his best friends and most important advisers. “His basic theory was Senator Boggs was beloved, but these were changing times,” he said. The pitch pretty much boiled down to this: “If Senator Boggs and I could just go down to the football stadium at the University of Delaware and people saw the two of us, they’d pick me.”

They did.

Now, though, at the opposite end of the cycle of his political life, Biden’s presidential candidacy is unavoidably an implicit argument against the very sort of youthful energy and not-gonna-wait rebellion that put him in a position of power in the first place.

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I was thinking about Delaware and 1972 a few weeks ago as I pulled in to park in a pecan orchard on the banks of the Little Pee Dee River in rural South Carolina. The first thing I saw on the door of a car was a big placard of a magnet bearing a rendering of the face of Pete Buttigieg. “We can’t look for greatness,” it said, “in the past.”

Up the road a short way, outside the Galivants Ferry general store, vendors in trucks and tents hawked sweet tea, pimento cheese sliders and six-buck chicken bog. A bluegrass band was picking on the porch not far from the lectern made to look like a stump. Sticking out in the growing crowd was a ruddy man wearing flip-flops and a “MAKE AMERICA GROOVE AGAIN” hat and carrying a sign saying he was a “REPUBLICAN FOR PETE.”

“Mayor Pete’s a selfless servant,” John Dabrowski told me. He’s 37, lives on Pawleys Island and works in sales at a brewery in Myrtle Beach. He’s not happy with Trump, whom he called “a bully.” After his wife, Peta, got turned on to Buttigieg by drinking wine and watching YouTube videos, Dabrowski, too, identified the alternative he was looking for. He thinks Buttigieg is the person who can beat the president. “We can’t put somebody that’s gonna ostracize Republicans that are moderates like me, fiscal conservatives but also super liberal in other, you know, social aspects.”

But what about Biden?

“No,” he said.

“He’s old guard. I believe in term limits, and Joe’s not gonna do it for me. I mean, we don’t need somebody that’s been doing it for all these years. I want somebody that’s gonna be around for the policies that they’re gonna write, OK? Mayor Pete, if we put in some type of green policy or some type of coal policy, he’s going to see the repercussions because he’s going to be around for it. Biden’s not gonna be around for 30 years to see the effects of his policies—and that matters to me. Doesn’t that carry more weight? I want somebody to be living with their policy.”

Dabrowski’s friend chimed in. Buttigieg “will be president,” said Ree Lawson, 54, a high school debate and public speaking teacher in Waccamaw. “It might not be this time.” But next time? Or the time after that? And after that? “We could go through four more cycles. He could run and still be a young man.”

On the docket for the evening at the Galivants Ferry Stump—where Democrats running for office have spoken since shortly after the Civil War—were four presidential candidates. First was Amy Klobuchar, and last was Bill de Blasio, who was a few days from dropping out—but in between was one heck of a Biden v. Boggs-like contrast.

Buttigieg, who was five years old when Biden first ran for president, bounded to his spot behind the stump, dressed in his uniform of dark slacks and a white shirt with the sleeves rolled up.

“This is a time to unify the American people. This is a time for ideas that are bold enough to get the job done and capable of bringing us together. And if you think about it, that’s how Democrats win, from John F. Kennedy to Jimmy Carter to Bill Clinton to Barack Obama. We win when we offer leadership from a new generation, with new ideas,” he said.

“Are you ready to break from the past?”

The cheer that went up from the people suggested that they were.

Then, though, led to the stump by a stomping, gyrating, instrument-waggling troupe from Florence, the energetic marching band from that city’s Wilson High, Biden was ready to respond. He spoke for his allotted 20 minutes, a faster, localized version of his standard spiel, citing another senator he once worked with (the recently deceased Fritz Hollings of South Carolina), stressing “honesty,” “dignity” and “decency,” and extolling Obama not as one of the series of fresh-faced Democrats who won the White House but as the president he served.

“Folks,” he said, “I’m just gonna say it. I don’t think we thank Barack Obama enough for the job he did as president!”

The audience roared. Biden built to a crescendo of his own. Grabbing hold of a past that never fails to invigorate him, he clenched his fist. He raised his voice. “Why in God’s name don’t we pick our heads up and remember who—this is the United States of America!”