Here are the ages the top presidential candidates will be on Inauguration Day, 2021: Bernie Sanders, 79; Joe Biden, 78; Donald Trump, 74; and Elizabeth Warren, 71.

If Sanders were elected and served two terms, he would be in office until age 87. If Biden did the same, he would serve until 86. If President Trump were reelected, he would serve until 78. If Warren served two terms, she would be 79.

There has never been a moment in American history in which so many of the top contenders for the presidency have been so old. There has never even been a time when two candidates over 70 ran against each other in a general election.

Before President Trump, there have been only two presidents who turned 70 in office: Ronald Reagan, who took office at 69 and left at 77, and Dwight Eisenhower, who took office at 62 and left three months after turning 70.

Now, if Sanders or Biden were elected, the president would turn 80 early in his first term. A second term would leave them heading toward 90.

That is totally uncharted territory in American politics.

Clearly, with Trump and the top three in the Democratic race all over 70, voters have decided that 70 is not a disqualifying age. But is 80? That is the question for Biden and Sanders.

That is not to say that an 80-year-old president will suffer from dementia, although that would certainly be more of a risk than a 60-year-old president. It is not to say that an 80-year-old president will suffer physical decline that will make performing the world's most demanding job more difficult, although that, too, would certainly be more of a risk than a 60-year-old president.

It is to say that there is a reasonable likelihood the president's age will be an issue throughout his or her time in office.

Reagan's second term, which began when he was 73, was filled with open discussion that he was losing it, that he was slowing down, that he was becoming senile.

The talk was quite common in 1987, with the release of the Tower Commission report on the Iran-contra affair. The report portrayed Reagan as aging and disengaged. "The issue of memory loss and age has been raised by observers of President Reagan and his role in the Iran-contra affair," the Washington Post wrote in March, 1987. A joke, playing on the old Watergate saying, emerged: "What did the president forget and when did he forget it?"

Edmund Morris, author of the idiosyncratic Reagan biography Dutch, observed Reagan closely during the second term. In 2011, Morris wrote that he never saw any signs of dementia in Reagan during those years. But:

Old age I saw; extreme fatigue, often; diplomatic occasions when his genius for telling the right joke at the right time deserted him; important meetings during which he read from cue cards like an obedient schoolboy. During one unhappy period, when the Iran-contra scandal coincided with prostate problems, the president was so withdrawn and confused that papers were surreptitiously drawn up by staffers concerned that he might have to be declared 'disoriented' and disabled under the 25th Amendment.

Of course Reagan, who seven years later would announce he had Alzheimer's, knew about the talk, and characteristically handled it with humor. "I've hit the point where I can recognize the three signs of the onset of senility," he said in 1988. "The first sign is loss of memory, and I can't remember the other two."

Back then, Reagan's gaffes and misstatements often led to discussions of his age. Much of that discussing came from Democratic-leaning commentators, while Republicans worried quietly.

Now, there is growing attention to Biden's gaffes and misstatements. Even though he was known as a "gaffe machine" throughout his 44 years in the Senate and the vice presidency, today, Biden's errors are discussed in the context of his age.

Just in the last few days, Biden recalled that he was vice president when the Parkland, Florida mass shooting occurred, when in fact it was the Sandy Hook shooting. On another occasion, he said, "We choose truth over facts." On yet another, he said the El Paso and Dayton mass shootings occurred in Houston and Michigan.

It was nothing terribly serious; anyone can misspeak. But there will be more, many more, if Biden's past is any guide. And they will lead to talk about his age.

But the age issue in 2020 is about more than just gaffes. It is a substantial question for the nation's voters. Any of the top contenders might well experience age-related problems in office, and voters need to consider that before electing them.

The race is in its early stages. It will inevitably become more passionate next year. When that time comes, age will be an issue.