For months, Senator Bernie Sanders brushed off questions about his age, offering a simple, six-word mantra to those who doubt he’s energetic enough to run for president: “Follow me around the campaign trail.”

But in the hours before the chest pains that led to an emergency procedure in a Las Vegas hospital, Mr. Sanders, 78, uttered a different, perhaps more telling, series of six words.

“Get me a chair up here,” he said, turning to his deputy campaign manager on Tuesday, before sitting down in front of the crowd of 250 gathered for a fund-raiser in a Persian restaurant. “It’s been a long day here.”

For months, Democrats have watched as a trio of septuagenarians commanded a majority of support in their crowded primary field: Former Vice President Joseph R. Biden Jr., 76, Senator Elizabeth Warren, 70, and Mr. Sanders, have consistently led in the contest to face President Trump, 73, next year.

Presidential campaigns always reflect the hopes and fears — or, as political strategists call them, the “kitchen table conversations” — of the voters who cast the ballots. And this year, along with health care costs and college affordability, stagnant wages and immigration, the contest also reflects another issue, one that strikes at the heart of a country where the highest share of the electorate will be older than 65 since at least 1970: How old is too old?

Voters, who have watched candidates through debate stages and state party dinners, on sweaty stages and speed-walking across the state fair, corn dog in hand, do not generally want to say there is a ceiling. No one is too old to be doing this. They just are not sure they would want to be keeping up such a rigorous schedule in their 70s. Would you?

Gerontologists and other experts in aging say there is simply no way to definitively address the question of an upper age limit on the rigors of the presidency.

“There’s no answer. It’s unknowable,” said Dr. Mark Lachs, co-chief of Geriatric and Palliative Medicine at Weill Cornell Medicine and NewYork-Presbyterian. “It’s true that rates of physical and cognitive impairment are age dependent but there’s all kind of variability.”

The averages paint a sobering picture: The average life expectancy in the United States is just under 79, according to the Centers for Disease Control and Prevention. Even as Americans live longer than ever before, about 85 percent of older adults have at least one chronic disease, and more than three-quarters have at least two.

But despite the statistics, one person’s 70 can be another’s 60, Dr. Lachs said. And being 70 years old, he added, is not at all what it used to be.

“The yardstick gets moved every decade because the country is aging and medical care becomes better,” he said. “Age should not be a disqualification for the presidency.”

That’s a message some voters happily believe, as they confront the realities of aging in their own lives. The number of Americans who plan to retire after age 66 has steadily ticked up over the past quarter century, with a quarter saying they do not plan to retire at all.

“Seventy is the new fifty,” said Halliestine Zimmerman, a retired accountant from Greenville, S.C., who celebrated her 70th birthday in June, a week before Ms. Warren did. “You don’t really think I am at the end of my life span, you think more you are in the middle. I know what it is I want out of my life, and you know how to get it.”

Ms. Zimmerman’s husband, whose age she would only give as “70 plus,” had a triple bypass several years ago and today he regularly plays tennis.

“He’s doing fantastic, so a stent doesn’t bother me at all,” she said, of Mr. Sanders. “I don’t think he will stop campaigning, I don’t think he should — remember a lot of people don’t make it to his age.”

But others worry that an older commander-in-chief would share the declines they have experienced in their own physical and mental abilities over the years.

Discussions of aging have been all-but-inescapable on the campaign trail. Since he entered the race, Mr. Biden has been dogged by questions about his physical fitness and condition — concerns he has tried to alleviate by bounding through parade routes and shaking dozens of hands in steamy summer weather. Mr. Sanders keeps a blistering campaign schedule that often includes multiple events in multiple cities each day. And supporters of Ms. Warren gush about her vitality, bragging about the hundreds of selfies she takes with supporters after each appearance.

“I was just amazed that when you first came out here, Senator Warren, that you ran up those steps the way that you did, and all this energy and stamina that you have,” Nikita L. Jackson, a Rock Hill, S.C., city councilwoman, said as she praised Ms. Warren to a crowd at a town hall event on Saturday.

None of the Democratic candidates have been particularly eager to delve into the details of their health. Aides to Mr. Sanders released a brief statement noting that he “was found to have a blockage in one artery and two stents were successfully inserted,” a fairly common procedure in the United States. Like his rivals, Mr. Sanders has not yet released his medical records, though all three have vowed to do so before the Iowa caucuses in February.

With little actual medical information, even minor irregularities in how candidates appear have prompted a flurry of age-related speculation. When Mr. Sanders hit his head on the edge of a glass shower door, his campaign explained that he had received a cut requiring stitches but stressed that he did not fall. Mr. Biden appeared to be moving his mouth in a strange fashion during the last debate, which led to questions about whether he wore dentures. At Mr. Biden’s campaign events, voters question whether his verbal missteps can be attributed to his age.

“He’s not as sharp as he might be,” said Carol Sobelson, 63, at a campaign event in Concord, N.H. “He’s done a lot for our country, he was a great vice president. He’s probably not my first choice.”

Health, or the perception of a candidate’s health, is unlikely to be off the table in a campaign against Mr. Trump. In 2016, his supporters spliced together video footage of Hillary Clinton coughing and Mr. Trump often questioned her stamina, particularly after she abruptly left a ceremony in New York honoring the 15th anniversary of the Sept. 11 attacks. Already, Mr. Trump has started questioning Mr. Biden’s energy levels, nicknaming him “Sleepy Joe.”

Nearly all Democrats prefer candidates in their 40s through 60s, according to surveys. When asked about the ideal age for a president, just 3 percent said the 70s, according to polling released by Pew Research Center in May. Other polls have shown that Americans express more discomfort with a candidate in their 70s than one who is gay, Muslim or an independent.

The two Democratic nominees who have won the White House since 1992 — Bill Clinton and Barack Obama — made generational change a key part of their winning campaign message. Both were the youngest in their primary fields.

Katie Glueck and Sydney Ember contributed reporting from New York. Thomas Kaplan contributed reporting from Reno, Nev., and Jennifer Medina contributed reporting from Greenville, S.C.