Biden, turning 80, faces an age-old question: How old is too old to be president?

It is one of the most replayed moments in the history of presidential debates.

Standing on a stage in Kansas City in October 1984, President Ronald Reagan was asked the question that was on most everyone's mind. He was the oldest president ever, and had been described as being "very tired" after an earlier debate. Did he have any questions in his mind about his ability to function if deprived of sleep during a national security crisis?

Reagan, 73 at the time, said, "Not at all." And, after scowling for a moment, he submitted to a flicker of a smile as he added, "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 question of how old is too old has also dogged President Biden, the oldest president ever to take the oath of office. Polls have found even Democratic voters express doubts about whether he's too old for the job. He told CBS 60 Minutes that his answer to the age question is, "Watch me," citing his schedule and energy level.

On Sunday, Biden turns 80. He has said he will be talking with his family during the next couple of months about whether to make good on an intention to seek a second term, at the end of which he would be 86.

"I'm a great respecter of fate. And this is, ultimately, a family decision," Biden told reporters earlier this month. "I think everybody wants me to run, but ... we're going to have discussions about it."

Reagan was the first president to face the 'how old is too old' question

In 1984, when Reagan deflected the age question on the debate stage with humor, everyone in the room laughed. That included Reagan's opponent, veteran senator and former vice president Walter Mondale, age 56 — even as he knew his slim chance of beating Reagan had just vanished. The incumbent went on to carry 49 states, a landslide victory.

But there is far more to the story than what is usually remembered. The question of Reagan's fitness had been very much in the air prior to that meeting in Kansas City. In an earlier debate, Reagan had seemed fuzzy on facts. When speaking of military uniforms, he struggled and came up with the word "wardrobe," a term more appropriate to his years as a movie star.

Speculation over Reagan's acuity loomed over the political conversation. Time magazine drew the candidates on cartoon horses running down a track with the headline: "A Real Race?"

Reagan's avuncular style lent itself to concealing uncertainty in a cloak of affability. But even on that stage in Kansas City, minutes after his marvelous quip seemed to bury the age issue, Reagan's closing speech raised questions.

Asked to sum up, he started describing a trip he and his wife Nancy had taken down California's coastal Highway 1 years earlier. The story meandered, much like that highway. Those present in the auditorium, including this writer, felt a rising sense of discomfort as Reagan continued on, aimlessly.

The moderator, NBC's Edwin Newman, finally interrupted and cut him off. And the room shifted back to its previous mood of assurance. Reagan was okay. Wasn't he?

Lapses in attention or effectiveness in his second term were sometimes attributed to his age. Voters, however, did not seem troubled. When Reagan's term was up, 40 states voted for his vice president, George H.W. Bush, to succeed him. Bush was 13 years younger than Reagan but very much part of the World War II generation.

Years after Reagan left office, the nation would learn he had Alzheimer's disease, a degenerative condition that eventually claimed his life. The revelation of the diagnosis reignited questions about whether there had been signs of early symptoms during his second term.

There is no upper limit on age for federal elected office holders, including presidents

Biden is the first octogenarian in the Oval Office. Only three others before him were in their 70s while in the White House. The first was Dwight Eisenhower, who turned 70 just before his term ended in January 1961, and the second was Reagan, who was 77 when he left.

Until Biden, Donald Trump had been the oldest president on his inauguration day. Trump was 74 when he left office, and has filed paperwork to run again. If he returned to office, he would be 82 by the end of that term.

While airline pilots face mandatory retirement at 65, federal law enforcement at 65, and judges in some states at 70, there is no age limit for elected federal office holders. The rule is that the voters decide.

As a reference point, check the U.S. Senate, where Iowa Republican Charles Grassley was just elected to another six-year term. He's already 89. Back in 2003, South Carolina Republican Strom Thurmond retired at 100.

But senators, no matter how important, are not presidents. They can take a day off, or even a week or a month. They are not the daily custodians of the nuclear codes, with all that implies.

The Constitution requires members of Congress to be 25 and presidents and vice presidents to be 35. That's it. No upper age limit. And apparently the participants at the convention that wrote that document in 1787 were not at all concerned with an upper limit. That may have been because they themselves were relatively young, averaging 42 years. The youngest was just 26, the oldest, Benjamin Franklin, was 81. (He was carried to and from the sessions in a sedan chair.)

There was a big age gap between Clinton and his rivals

Age was an implicit contrast in the 1992 election, when Bill Clinton challenged incumbent President George H.W. Bush. Clinton was a youthful 46 when he was first nominated, and was still widely perceived as the "Boy Governor" first elected in Arkansas when he was just 32.

Bush was 22 years his senior. At the time, it was the widest age gap between the two major parties' nominees since before the Civil War.

In 1996, when President Bill Clinton was seeking re-election at age 50, the age issue was raised in another presidential debate in 1996. His Republican challenger was Bob Dole, a longtime senator who was then 73.

"I don't think Senator Dole is too old to be president. It's the age of his ideas that I question," Clinton said.

Dole did not seem overly ruffled by references to his age, although he did generate anxiety midway through the campaign when he leaned on a flimsy railing and fell a short distance from the stage at a campaign rally.

Candidates need to be mindful of every public move they make. That image of Dole's tumble has since served as a reminder that this imperative applies especially to those of a certain age.

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