Sen. Bernie Sanders debates his Republican challenger for the Senate, Lawrence Zupan, in late October on VPR/Vermont PBS.

[L]ast week, Sen. Bernie Sanders sent his supporters a message saying, โ€œif I am the best candidate to beat Donald Trump, then I will probably run.โ€

Sanders asked for $3 contributions, complained about opposition already being organized against him, and claimed that he was the one who has โ€œthe agenda the American people want,โ€ and the one that can win.

Sounds like heโ€™s running.

But on the last day of the old year, Sanders got one-upped. Sen. Elizabeth Warren, of next-door Massachusetts, announced that she was running.

Warren, by something close to common political consent, is Sanders main competitor for the support of the Democratic Partyโ€™s โ€œprogressiveโ€ wing, as opposed to its โ€œestablishment.โ€ Whether this distinction is meaningful โ€“ whether the two factions differ over policy or merely over temperament โ€“ is open to question.

But thatโ€™s a discussion for another day. For now, what is important is that if Sanders runs the two of them will be battling from the outset for the same slice of the Democratic primary/caucus electorate.

Each has advantages. Sanders has a committed following from his challenge to Hillary Clinton. He has money and the ability to raise more. He can reassemble much of the organization that helped him come close to the nomination in 2016. Heโ€™s better known than Warren.

Her advantages are that sheโ€™s a woman running when Democratic women are energized, feeling flush about electing so many women to Congress last year. She comes across as calmer than Sanders.

trump commander in chief
President Donald Trump addresses service members in November 2017 at Yokota Air Base in Japan. On Jan. 9, 2017, President Trump signed an executive order that allows transitioning service members and veterans to receive up to a year of mental health care from the Veterans Affairs Department after discharge from the service. Photo by Airman 1st Class Juan Torres/Air Force

And come November 3, 2020, Election Day, she will be 71 years old.

Thatโ€™s older than most presidents are when they first win office. But itโ€™s seven years younger than Sanders, a reality that could and perhaps should be a factor in this yearโ€™s campaign.

Thatโ€™s because for the first time, three people in their late 70s — Sanders, former Vice President Joe Biden and former New York Mayor Michael Bloomberg — are on the verge of mounting credible campaigns for president. If any of them gets elected next year, he will turn 80 before his first term ends. That may be too old.

No president has been 80. Most presidents have been in their 50s or 60s when they left office. George Washington was 65. Grant was 54. Teddy Roosevelt was 50, and Dwight Eisenhower, the oldest until Ronald Reagan, was 70. Barack Obama was 55.

Reagan left office a couple of weeks before his 78th birthday. Five years later, he announced that he had been diagnosed with Alzheimerโ€™s Disease. He was then 83. Thatโ€™s how old Sanders will be when the next presidential term ends. Biden will be 82.

This is a delicate and difficult subject, but unavoidable. Neither Biden nor Sanders is likely to be afflicted by dementia over the next four years. But people of their age (full disclosure: that includes me) are notably more likely to suffer from it than are people in their 50s and 60s.

The correlation between age and dementia is irrefutable. Different studies reach different conclusions. But one Italian survey found that 13.7 percent of people in their 80s had a form of dementia. Alzheimerโ€™s researchers in Britain concluded that โ€œa personโ€™s risk of developing dementia rises from one in 14 over the age of 65 to one in six over the age of 80.โ€

No one should doubt that the comparatively youthful Donald Trump (heโ€™ll be 74 on election day) will remind voters that if they choose one of these older gentlemen they are taking the risk that they will have a demented president in a few years.

Irresponsible? Outrageous? Maybe. But not conclusively incorrect. And certainly not beneath the dignity of todayโ€™s Republican Party.

All this creates a dilemma for Democrats, both party leaders and rank-and-file voters. Right now, Sanders, Biden, and Bloomberg appear hale and healthy. By all indications, their minds are as sharp as ever they were. As people live longer โ€“ and many of them stay fit longer โ€“ the whole country has been bombarded by slogans about how 50 is the new 35, 65 is the new 40, 70 is the new 50, or whatever.

There is something to that, what with more people trying to eat healthy diets, hitting the gym and following โ€“ sometimes almost worshipping โ€“ various exercise regimens. Sanders and Biden have loyal followings (if Bloomberg does, itโ€™s not visible), who might be expected to argue that it is unfair to these candidates even to mention their age.

Suppose Democratic nominee Sanders caught a cold two weeks before the election. Or stumbled. Social media would have him at deathโ€™s door and maniacal within the hour. Thatโ€™s what happened to a mildly ill Hillary Clinton in 2016.

Yes, a 45-year-old candidate can catch cold or stumble, too. But it wouldnโ€™t be as damaging.

None of this ends the Sanders campaign before it starts. It is a factor he will have to deal with, not the one that breaks the deal. But it wonโ€™t go away by itself.

Jon Margolis is the author of "The Last Innocent Year: America in 1964." Margolis left the Chicago Tribune early in 1995 after 23 years as Washington correspondent, sports writer, correspondent-at-large...