Analysis by Zachary B. Wolf, CNN

Joe Biden was a precocious young buck when he took the oath of office to become a US senator in 1973, among the youngest people ever to sit in that chamber.

Now 76, he's still in the game and would be the oldest-ever first-term President if he's elected in 2020. And he's taking questions about his age seriously.

"Just like when I was 29, was I old enough? And now, am I fit enough? I'll completely disclose everything about my health. I'm in good shape," Biden said told the New Hampshire newspaper The Laconia Daily Sun last week.

Biden is nearly four years older than President Donald Trump, who became the oldest first-term President when he was inaugurated at 70.

Bernie Sanders, at 78, is also in the mix. And so is Elizabeth Warren, who is now 70, although you wouldn't know it from the way she bounds around the campaign stage and runs, literally, to events.

So the top three Democrats in recent primary polls are all septuagenarians, even as the party tries to market itself as the progressive and diverse alternative to old, white Republicans.

Nobody younger than 70 is in double digits in the most recent national Democratic primary poll, from ABC News and The Washington Post.

Any of them -- Warren, Sanders or Biden -- would be the fifth straight baby boomer president. Barack Obama, born in 1961, is toward the tail end of the baby boom, according to Pew).

But while they seemed young when Bill Clinton defeated President George H.W. Bush in 1992, now they're in the twilight of their careers. Even if they can get a fifth president, the baby boomers will still be far behind the Greatest Generation, which arguably includes seven presidents if you squeeze Lyndon B. Johnson (1908) and Jimmy Carter (1924) into the same generation, which Pew does.

Yet 40% of Democrats and Democratic-leaning adults told ABC News and The Washington Post that someone under 70 would have a better chance against Trump, compared with just 3% who said someone older would have a better chance.

That makes it even more interesting that as the Democratic primary has proceeded, none of the younger coterie has broken into the top tier.

South Bend, Indiana, Mayor Pete Buttigieg is among the Democrats who have called for generational change as part of his pitch to voters.

"When you take a look at me, my face is my message, right? A lot of this is simply the idea that we need generational change, that we need more voices stepping up from a generation that has so much at stake in the decisions that are being made right now," he said back in March.

Age might affect Biden more than others. Voters already essentially accepted Trump's age when they elected him, and now he's running as an incumbent. Sanders was a new voice to many Americans in 2016, although he's in his second run now.

But Biden has been at the national front of US politics for decades. He had to explain his opposition to busing in the '70s and '80s after the first Democratic debate and who has had to reckon with his treatment of Anita Hill at Clarence Thomas' Supreme Court confirmation hearings in the '90s.

Sanders and Warren are native progressives and more in line with the young energy of the party, despite their generational status as boomers.

"Maybe it takes a woman of a certain age to say, 'I know how to fight, I know how to win and that's exactly what I'm going to do,' " Warren said in New Hampshire.

In January Pew published a detailed sorting of the social and political views of Americans by generation and found that millennials -- Buttigieg and Rep. Tulsi Gabbard of Hawaii are on the cusp of that generation -- are much more progressive than their older voting-age fellow citizens. And the Generation Z Americans, those born beginning in 1997 and now coming into their voting power, are in line with or even more progressive than millennials.

Just 49% of baby boomers said in that Pew survey that the government should do more to solve problems, compared with 64% of millennials and 70% of those in Generation Z.

Just 48% of baby boomers said increasing racial and ethnic diversity was good for society. Compare that with 61% of millennials and 62% of Generation Z.

That's also a reflection of the fact that the country itself is becoming more diverse. That's had an effect on the makeup of Congress and on the full Democratic field. Eventually, it will be seen in the top tier of a presidential ticket. Just maybe not yet.