How Old Is Too Old to Be President? An Uncomfortable Question Arises Again.

When Dwight D. Eisenhower weighed the pros and cons of running for a second term, one factor that concerned him was his age.

Arguing against a re-election campaign in his mind, he wrote in his diary in November 1954, was the need for “younger men in positions of the highest responsibility” at a time of “growing severity and complexity of problems that rest upon the president.”

He was 64 at the time.

Today the two leading candidates for his old job clock in at 77 and 81. Barring an unforeseen political earthquake, America seems destined to have a commander in chief well past typical retirement age for years to come no matter who wins in November. Donald J. Trump would be 82 at the end of the next term, and Joseph R. Biden Jr. would be 86.

Aging today, of course, is different than it was in the 1950s, and Eisenhower did decide to run again, serving out a second term leading an administration that historians credit as formidable. But he experienced multiple serious health scares in office that tested his Cold War presidency, and it seems reasonable to assume that the country could be confronted with similar issues between now and January 2029, when the next term will expire.

The issue of age was thrust back onto the front burner with the special counsel report on Mr. Biden’s handling of classified information that described the president as a “well-meaning, elderly man with a poor memory” who had “diminished faculties in advancing age.” The report came the same week that Mr. Biden on two occasions referred to European leaders who are, in fact, dead as if they were still around and mistakenly called the president of Egypt the president of Mexico.

Mr. Trump quickly sought to capitalize on the special counsel report, issuing a statement through an aide calling Mr. Biden “too senile to be president.” But Mr. Trump has suffered his own bouts of public perplexity lately, confusing the leaders of Hungary and Turkey, warning that the country is on the verge of World War II, saying that he defeated Barack Obama instead of Hillary Clinton and referring to his Republican primary challenger, Nikki Haley, as if she were Nancy Pelosi, the former House speaker.

As a matter of politics, age has been a bigger liability for Mr. Biden than for Mr. Trump, according to polls, perhaps because of the president’s physical presentation, particularly the shuffle when he walks. Mr. Biden, who unlike Mr. Trump exercises regularly, has agreed that age is a legitimate issue to consider but grew incensed over the report by the special counsel, Robert K. Hur, and made a last-minute decision to summon cameras to the White House for a feisty nighttime pushback.

“Biden clearly finds the conversation about his health and age exasperating,” said Jonathan Darman, author of “Becoming FDR,” about the health challenges of President Franklin D. Roosevelt. “This is understandable, particularly given Trump’s own advanced age, his apparent confusion and his frequent lapses of memory. But even if, as Biden and his aides insist, he is in excellent physical and mental health, he owes it to the country to have a frank and robust conversation about the topic.”

Neither candidate seems eager for that. Both have issued reports from doctors stating that they are in good shape, but neither has answered questions about their health at length. While the White House physician has been made available to reporters by previous presidents, Mr. Biden has not seen fit to order his doctor to respond to detailed queries.

Even assuming both are fit for the presidency at this point, the harder question for voters to evaluate is whether they will be in five years. And the dilemma for the country would be what to do if a president slips mentally or physically in a way that affects his ability to do the job but will not admit it or voluntarily step aside.

History suggests that presidents do not willingly give up power no matter how impaired they may be, and the constitutional mechanism for removing them enshrined in the 25th Amendment is politically problematic. Among other things, it requires a vice president and majority of the cabinet to declare that a president is “unable to discharge the powers and duties of his office,” which loyal appointees may be reluctant to do if the president does not agree. Even if they did, a defiant president could appeal to Congress, requiring a two-thirds vote by both houses to sustain his removal.

Some of Mr. Trump’s own cabinet members when he was president contemplated invoking the 25th Amendment to unseat him, but his vice president, Mike Pence, refused to go along. The 25th Amendment provides an alternative: A panel created by Congress could declare a president unable to serve, but lawmakers have never formed such a body. When Representative Jamie Raskin, Democrat of Maryland, tried to create a bipartisan panel of outside experts during Mr. Trump’s presidency, the initiative went nowhere.

The issue has arisen in different forms at various points in American history. President James A. Garfield was shot by a would-be assassin in 1881 and lingered for 80 days before dying, during which time he was hardly in shape to be running the country. Likewise, President Ronald Reagan was shot in 1981 and hospitalized for nearly two weeks, although his staff labored to create the perception that he was able to govern from bed.

After Eisenhower’s rumination on age in his diary, chronicled by biographers like Jeffrey Frank, the general-turned-president suffered a heart attack in 1955 and underwent surgery in 1956 for an obstruction caused by Crohn’s disease before nonetheless winning re-election. In 1957, he had a small stroke but completed his term in 1961. Like other presidents, he convinced himself he was uniquely suited to the White House and ran again.

Eisenhower overruled aides who wanted to hide his condition from reporters, instructing his staff to “tell them everything.” The health issues “kept no one from voting him a second term,” noted Richard Norton Smith, a former director of the Dwight D. Eisenhower Center in Abilene, Kan. “Indeed, that wound up educating people about the modern treatment of heart and other diseases once presumed debilitating — even if Ike found distasteful public depictions of his internal organs.”

Roosevelt was always struggling with the politics of health, forced to convince the country that he was up to the presidency when he first ran in 1932 despite having lost the use of his legs because of polio. Roosevelt clearly proved capable despite the disease, and Mr. Darman argues in his book that it made F.D.R. a better, more empathetic and determined leader.

By the time he ran for a fourth term in 1944, however, Roosevelt was so drained and diminished that his own doctor did not believe he would survive the term, which proved prescient. “Knowing what we know now,” said Mr. Darman, “his decision to run that year is hard to defend. Roosevelt’s aides told the country his health was good, but anyone who was in close contact with him at that point in his presidency could see that his physical stamina was dramatically reduced.”

The most famous and serious presidential disability crisis came when Woodrow Wilson collapsed during a cross-country train trip promoting his League of Nations in 1919. Later felled by a stroke, he was hardly in shape to govern, leaving his wife Edith Wilson and a handful of aides to shield him from public view and effectively manage his presidency for nearly a year and a half.

“He really should have stepped down, at least temporarily,” said Rebecca Boggs Roberts, author of “Untold Power,” a biography of Edith Wilson. “He hated the idea, and his wife, Edith, chose to protect his feelings over the needs of the nation, and basically did his job for him while lying to everyone about how sick he was. That is not how democracy is supposed to work — no one elected Edith to anything.”

Such extensive subterfuge seems impossible in today’s hyper-accelerated political and media world. But White House aides still do what they can to cover for ailing presidents. In Reagan’s later years in office, some were so worried about his mental state that they thought they might have to orchestrate the invocation of the 25th Amendment, as revealed in “Landslide,” by Jane Mayer and Doyle McManus.

After closely studying him, the aides decided he was still capable of handling the job. But nearly six years after leaving office, Reagan disclosed that he had been diagnosed with Alzheimer’s disease, and many have debated since whether the effects of that may have been taking hold while he was in the White House.

Reagan defused concerns about his age during his 1984 re-election campaign with humor. After a halting performance at a debate against his Democratic challenger, Walter F. Mondale, left many voters with questions, Reagan, then 73, joked at his next debate: “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.” Even Mr. Mondale laughed and said later that he knew at that moment he had lost.

Until recently, Reagan was the oldest president in American history, leaving office just a couple of weeks before his 78th birthday. Mr. Biden was older on his first day in office than Reagan was on his last, taking over the title of oldest president. If Mr. Trump wins in November and finishes his second term, he will overtake Mr. Biden for that distinction.

Mr. Darman said the lesson he learned from history was that Roosevelt dispelled worries about his health with a vigorous campaign schedule. “Americans today have doubts about Biden’s ability to handle the demands of the presidency,” he said. “The only way for him to address those doubts is to do what Roosevelt did — get out in public and show the country that he is up for four more years.”

Mr. Trump, too, will have to quell concerns about his cognitive health, something that was a serious enough worry while he was in office that many of his aides privately believed he was not fit. His own second White House chief of staff bought a book by a series of mental health experts to try to understand Mr. Trump. But Mr. Trump has many other issues that may overshadow his health, most obviously the 91 felony criminal counts against him.

As the general election contest takes shape, the emerging choice between octogenarian and septuagenarian may be unique in American history. But it may not be the last. Given longer life spans and advances in medical science, Mr. Smith said, “we had better get accustomed to older presidents.”