Is Age Only a Number, Even When You’re Running for President?

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 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,” Carol Sobelson, 63, at a campaign event in Concord, N.H., said. “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.”