The Democratic presidential nomination was open, and it was the vice president's turn — at long last.

His loyalty to the administration was unquestionable, his credentials were unimpeachable and his politics aligned with the dominant forces within the party. There was no glaring reason for Democrats not to turn to him, except for this: He was old.

This was in 1952, more than a half-century ago, when Alben W. Barkley, Harry's Truman's 74-year-old vice president, came to the Chicago convention convinced that his moment at the top of the ticket was finally at hand, only to be greeted by union leaders who had the power to thwart him and who'd decided to do just that.

"We can't sell Barkley to labor,” one of them announced, "not because of his record, but because of his age."

And that was that. Lashing out at "certain self-appointed political labor leaders," Barkley withdrew from the race and gave up his dream. Two years later, intent on disproving the notion that he was too frail for national politics, Barkley won back his old Senate seat from Kentucky, but he soon suffered a fatal heart attack while delivering a speech. His death came nearly a year before his term as president would have ended.

When it comes to serving as president, the Constitution sets only a minimum age, not a maximum one. But Barkley was butting up against a notion that prevailed until recently in American history: that there was an informal limit on how old a president could be, and that it was — roughly speaking — the late 60s.

This had something to do with life expectancy and with the widespread adoption by the early 20th century of 65 as a standard retirement age. There was also the cautionary case of William Henry Harrison, who was 68 years and 23 days old when he was inaugurated as president in March 1841 — and 68 years and 54 days old when he died a month later.

The three septuagenarian Democrats now running — Bernie Sanders (78), Joe Biden (76) and Elizabeth Warren (70) — and the current 73-year-old occupant of the White House all demonstrate that it's no longer remarkable for someone over 70 to seek the presidency or even to win it. Clearly, that old informal age cap has been raised, but just how high remains unclear, and it looms as a potentially crucial variable as Democrats choose their nominee.

The standards changed because of the 1980 election. Ronald Reagan was the next-in-line Republican candidate, the runner-up in the 1976 nomination fight that went all the way to the convention. But he was also approaching his 70th birthday, a number that was still thought to be prohibitive when it came to launching a campaign. Columnists Roland Evans and Robert Novak declared him "by an objective reckoning, too old to run for president" again.

Reagan went ahead and ran anyway, and questions about his age featured prominently in the campaign. At one point, 64-year-old Sen. Gaylord Nelson, a Wisconsin Democrat, announced that Reagan had inspired him to make his own re-election campaign in 1980 his last. "If I am elected and serve out my term," Nelson said, "I would be as old as Reagan is now. And that's too old to run."

President Ronald Reagan gestures as he participates in a White House East Room news conference on Feb. 21, 1985 in Washington. Ron Edmonds / AP file

It was the growing power of television that allowed Reagan, a trained actor, to assuage these fears with command performances in highly visible moments.

There was the Republican primary debate where he dusted off an old movie line — "I am paying for this microphone!" — to turn the New Hampshire race in his favor. And there was his single showdown with President Jimmy Carter, when Reagan used a pair of devastating lines — "There you go again" and "Are you better off today than you were four years ago?" — to transform a close contest into a landslide. Critics said he was mangling facts, but the Reagan that millions of Americans encountered in 1980 was sharp, confident and quick-witted.

Four years later, running for re-election at age 73, Reagan's lead over Democrat Walter Mondale was briefly threatened when he appeared confused during an October debate and gave a series of halting and meandering responses. With the country watching closely in the next debate, Reagan returned to his form and famously defused the matter of age with a joke about not making an issue of his 56-year-old opponent's "youth and inexperience."

The Reagan years hardly ended the debate over age and the presidency. His second term produced further concerns about possible mental decline and later, after he left the White House, came his Alzheimer's diagnosis. Nonetheless, Reagan remained in office for two full terms and departed just short of 78 years old with strong popularity.

When it comes to the question of age, his presidency absolutely shifted the goal posts.

Since Reagan, three more septuagenarians have been nominated for president: 73-year-old Bob Dole in 1996; 72-year-old John McCain in 2008; and 70-year-old Donald Trump in 2016. There was also 68-year-old George H.W. Bush in 1992 and 69-year-old Hillary Clinton in '16; both would have governed as septuagenarians had they won. Now there's the possibility that in 2020 both parties will field candidates who are 70-something.

But this campaign may still test limits. A recent NBC News/Wall Street Journal poll found that more than six in 10 Americans say they'd have reservations about a candidate over 75 years old. Neither party has yet nominated someone that old, but both Biden and Sanders are past that threshold. Assuming good health, they'd each reach 80 in a first term — uncharted territory for a commander in chief. Will this matter to voters?

Jimmy Carter, 95, said this week he hoped there would be an age limit for the presidency and confessed he couldn't have done the job at 80.

The question is particularly key when it comes to Biden, the Democratic front-runner. On policy, he's largely in line with today's Democratic orthodoxy, and polls continue to show him outpacing his rivals in match-ups against Trump. In many ways, he's the logical heir to his party's nomination. But in the most widely seen moments of the campaign, the televised debates, Biden has so far delivered shaky performances that have aroused concerns about whether his age is showing too much.

The example of Reagan is worth returning to here. Because he excelled when the spotlight was brightest, he could convince voters that his age didn't matter. If Biden continues to struggle in the spotlight, he risks voters deciding that his age does.