Today, 76-year-old Joe Biden is leading the polls for the Democratic nomination, followed by Senators Bernie Sanders, 78, and Elizabeth Warren, 70. Both Mr. Sanders and Mr. Biden would be older going in than Mr. Reagan was coming out, and Mr. Biden, in particular, has established something of a pattern of blunders that has invited questions about his mental fitness to hold the most powerful office on the planet. And given that the average life expectancy for white males was 76.4 years in 2017, concern about physical longevity has also reared its head.

[The big debates, distilled. This guide will put in context what people are saying about the pressing issues of the week. Sign up for our new newsletter, Debatable.]

WHAT PEOPLE ARE SAYING

Yes, there should be an age limit

For Mr. Carter, putting an age cap on the presidency is a matter of guarding against the cognitive decline that naturally attends old age. “You have to be very flexible with your mind,” he said, adding:

You have to be able to go from one subject to another and concentrate on each one adequately and then put them all together in a comprehensive way. … The things I faced just in foreign affairs, I don’t think I could undertake them if I was 80 years old.

Caitlin Schneider has expressed a similar view in the progressive outlet Splinter, floating a maximum age of 65. She writes:

Some might argue that the sheer premise of this story is ageist, but being president is (theoretically) an incredibly taxing job! To question whether we ought to have an age cap on candidates isn’t designed to disparage the old, but to take a long hard look at the job at hand. If the rules say a 34-year-old can’t do it, it’s worth asking: can an 80-year-old?

Andrew Ferguson writes in The Atlantic that an age limit may be necessary to break up the gerontocracy — rule by the old — within the Democratic Party, whose leadership is on average 24 years older than that of the Republican Party. He writes:

There is a huge gap between where the energy and creativity of the party lie, with a group of dynamic activists and House members in their 30s and even their 20s (thank you, Alexandria Ocasio-Cortez), and the ruling class of 70-somethings layered far above like a crumbling porte cochère. … The trick for old folks is to adjust their search for purpose and meaning as they follow nature’s course and give way to their juniors.

No, there shouldn’t be an age limit

Calls for disqualifying older candidates in the absence of specific evidence of poor health is discriminatory, argues Ashton Applewhite, the author of “This Chair Rocks: A Manifesto Against Ageism.” Statistical relationships between age and cognitive decline, she says, tell us nothing about individuals running for president, who are de facto atypical. She writes:

Eighty-year-old senators are healthier than the average octogenarian; many exhibit astonishing intellectual powers and physical stamina. Nor is Bernie Sanders the average 78-year-old. Clearly he should undergo a physical exam by nonpartisan authorities and make the relevant results public, as should all presidential candidates. … But generalizations about the capacities of older people are no more defensible than racial or gender stereotypes. Period.

Duke University professors James Chappel and Sari Edelstein , who study the culture of age, write in The Washington Post that research shows elderly people are more cognitively capable than common prejudices suggest and in some cases have more to offer than their juniors: Nelson Mandela and Winston Churchill, for example, were effective leaders through their 70s. Dr. Chappel and Dr. Edelstein write:

Rather than contemplate the disqualification of candidates because of their advanced age, we would do well to consider how older candidates might bring a heightened awareness to issues of inequality and discrimination, a wealth of policy expertise, and the adroitness and diplomacy that comes with years of experience in the government.

Moreover, as John Della Volpe, the director of polling at the Harvard Institute of Politics, pointed out to The Times, a candidate does not have to be of a particular generation to represent its interests. Support for Mr. Biden and Mr. Sanders, for example, is deeply polarized by age, with Sanders leading among those under 35.

It’s the age minimum that should go

Americans under the age of 35 are second-class citizens, the writer Osita Nwanevu has argued in Slate. The country’s patchwork of age restrictions for federal and state office was born out of the founding fathers’ half-baked assumptions, based on personal experience, about youthful incompetence — a patently illogical prejudice , he says, given that 12 of the delegates at the Constitutional Convention, including Alexander Hamilton, were under 35. (Although since there was little debate on the issue, it’s possible the logic was more evident to even those younger delegates at the time.)