The entrance of former Vice President Joe Biden into the 2020 White House race adds yet another septuagenarian into the crowded field of contenders.

Of the 20 Democrats and two Republicans who are running for president, six are nearing or over the age of 70.

If Sen. Bernie Sanders, I-Vt., wins, he would be 79 by the time he reaches the White House, becoming the oldest president in history; Ronald Reagan was 77 years and 349 days on the day he left office. President Trump, who turns 73 this summer and has long batted back scrutiny about his health, is the oldest president when first sworn in.

The other older Democrats are Biden, who would be 78 when first sworn in, Sen. Elizabeth Warren, D-Mass., who would be 71, and Washington Gov. Jay Inslee, who would turn 70 shortly after being sworn in. Bill Weld, the former Massachusetts governor challenging Trump for the GOP nomination, would be 75.

"I think we're asking for trouble," Dr. David Scheiner, who was Barack Obama's doctor for more than two decades before Obama became president, said of the number of older candidates in the race. "We don't know what their health is. They look spry but I don't think that's enough. You can't accept it at face value."

Candidates who are older are more likely to face scrutiny about their health. According to the Social Security Administration, men who have reached age 70 can expect to live roughly 15 more years, and women can expect to live about 17 more years, but lifestyle and health factors can shift the calculus. And as people get older, the probability of illness, disability, and death increases, as do the chances of cognitive impairment.

[Opinion: Byron York: Face it: Biden and Bernie are too old to be president]

Sanders and Biden, who are the oldest candidates, poll at the top of the Democratic field. Christopher Devine, assistant professor of political science at the University of Dayton in Ohio, said that he doesn’t see age as being a deciding factor for voters. "If you're older you emphasize experience, and if you're younger you emphasize being a being a fresh face," he said. "You play to your strengths."

Sanders enjoys a devoted following among younger voters. When asked about his age during a Fox News town hall, he replied that "my health is good" and "I continue to have my endurance." He added: "It's not whether you're young, it's not whether you're old. It is what you believe in."

S. Jay Olshansky, a leading researcher on aging from the University of Illinois at Chicago, argues that a person's age should not matter when they're being considered for powerful positions. "As far as I can tell, the candidates that are going to be running are functioning at a very high level cognitively," he said. "I don't see any reason why the age of these individuals should be taken into consideration, period."

But questions about age are unlikely to go away, especially because of the stark contrast between candidates. One of Sanders' rivals, Pete Buttigieg, the 37-year-old mayor of South Bend, Ind., is less than half his age.

In past elections, candidates have weaponized opponents' ages, either to argue that they lack experience or, alternatively, that they are too old to take on the grueling work of running for office and eventually running the country.

In the run-up to the 2016 election, Democratic nominee Hillary Clinton drew widespread scrutiny after she stumbled at a 9/11 Memorial event. Her staff later said she had pneumonia, but Trump said that she "doesn't have the stamina" to be president.

Trump has faced his own scrutiny from critics who point to his late-night tweets and an instance in which he slurred his speech, as well as the weight he's gained since taking office, as signs that he's too old and is suffering from cognitive decline. Revelations that he dictated his family doctor's letter declaring him to be "the healthiest individual ever elected to the presidency" reinvigorated opponents, as has the scarcity of information the White House released in his most recent medical checkup.

Scheiner, Obama's former doctor, thinks Trump's situation underscores that candidates should be evaluated by an independent group of experts prior to an election and undergo a neurocognitive test. Presidential candidates since Reagan have released some of their medical records to the public, but no law demands their release and no candidate has disclosed a full medical history.

"The business of keeping these things secret has really gotten out of hand," Scheiner said.

Sen. John McCain, R-Ariz., made more than 1,500 pages of medical and psychiatric records available to reporters when he ran for president against Obama because he had a history of cancer and was a prisoner of war. Still, his opponents alluded to his age by running ads warning voters that running mate Sarah Palin was "one heartbeat away" from the presidency.

At the same time, history has shown that candidates are judged positively when they field questions about their age with aplomb. Reagan, who was almost 70 when he became president, drew laughs when he said he would not use "my opponent's youth and inexperience" against the 56-year-old Democratic nominee Walter Mondale.

One of Bill Clinton's lines about Bob Dole, who was more than 20 years his senior, was considered to have staying power: “I don’t think Sen. Dole is too old to be president," he said. "It is the age of his ideas I question.”

Ultimately, age doesn't rule out a two-term presidency, and people who run for office have lived to be older than expected. McCain died last summer at the age of 81, after what would have been the duration of both terms if he had been elected.

Reagan served two terms before dying at 93 in 2004. Gerald Ford died at 93 and George H.W. Bush died at 94. Jimmy Carter, 94, is still living.

Devine from the University of Dayton said it would be difficult to imagine a scenario in which any candidate reveals poor health under the current tradition, in which candidates have their personal doctors write letters signing off on their fitness for duty.

"If they are in the race and they are committed to it, they are either in good health or they will find some way of portraying themselves as being in good health," he said.