Whatever their differences, the two front-runners in the Democratic presidential race, Joe Biden, born November 20, 1942, and Bernie Sanders, born September 8, 1941, share one common trait: They are too old to be president.

Biden, leading the Real Clear Politics average of polls, will be 78 years old on inauguration day 2021. Sanders, number two in the field, will be 79. Both would be older upon taking office than Ronald Reagan was when he left office after two terms.

Voters are clearly open to older candidates. President Trump, born June 14, 1946, is the oldest president ever to take office — 70 years old on inauguration day, a few months older than Reagan when he took power. And of course, Trump defeated Hillary Clinton, born October 26, 1947, who, had she won, would have been the same age as Reagan, 69, upon taking office.

Americans elect a president with the understanding that he or she might serve eight years. If that were the case with a President Sanders, he would be 87 years old on leaving office, and a President Biden would be 86.

There's no doubt both are vigorous men. But having a president pushing 90 would be a new experience in American politics.

[Related: Democratic Party future? The two 2020 front-runners are white men with a combined age of 153]

According to actuarial tables maintained by the Social Security Administration, the life expectancy of a 70 year-old man is 14.30 years — enough time to serve two terms and move on to physical decline. The life expectancy of a 78 year-old man is 9.33 years — enough to last two terms and not a lot more. The life expectancy of a 79 year-old man is 8.77 years — barely enough to make it out of the White House.

Of course, each man might live to 100. None of us knows. But given their age, the issue is more than just whether each candidate might be a voice from the past, or out of touch with the today's concerns. The issue is whether they are simply too old to handle the rigors of the presidency.

There's no doubt the Republican candidate, President Trump, will be 74 years old on the next inauguration day. What is unknown is whether Democrats will choose a candidate who is even older, or whether they will give voters a generational choice. First, though, the Democratic Party has to go through a generational reckoning of its own.

The party has a host of candidates who are in prime age for the presidency. In third place in the Real Clear Politics average is Kamala Harris, born October 20, 1964, who will be 56 on inauguration day. Next is Beto O'Rourke, born September 26, 1972, who will be 48 (just a bit older than Barack Obama was when he took office).

Next is the candidate who would be the oldest in the Democratic race were it not for Sanders and Biden. Elizabeth Warren, born June 22, 1949, will be 71 on inauguration day — older than Trump was when he took the oath of office. But given the experience of Reagan and Trump, and the candidacy of Clinton, Warren appears within the accepted range of presidential age.

Continuing down the Democratic field, there is Cory Booker, born April 27, 1969, who will be 51 on inauguration day. Then Pete Buttigieg, born January 19, 1982, who will turn 39 the day before the inauguration and is the subject of endless profiles noting that he would be the first millennial president. Then Amy Klobuchar, born May 25, 1960, who will be 60 on inauguration day. After that, all the lower-ranking candidates are in the same prime age.

All are in the zone. And all stand apart from Sanders and Biden.

The former vice president has taken a lot of heat for things he said and positions he held 30 and 40 years ago. He's also in the middle of a #MeToo mess over his too-familiar way of touching women. Sanders, too — the man who honeymooned in the Soviet Union — can seem the product of an earlier era.

But there is a much simpler point to be made about Biden and Sanders. They are too old to occupy the nation's highest office, not because they are out of touch or because their records are out of line with today's sensibilities, but because they are just too old.

For those who respond, "Well, what about Trump?" remember: If the president serves two terms, he will leave the White House at 78, the oldest ever in office. But that is the age Biden would begin his presidency. And Sanders is a year older.

Will Democrats will realize that and will nominate a candidate who draws a clear generational contrast with President Trump? In another age, Biden or Sanders might have been a plausible nominee (although both have tried and failed). But not now.