Why are the Democrats running for president so old? Blame the Clintons.

There are five Democrats who have either declared or are thinking about running for president. Three — Joe Biden, Bernard Sanders, and Jim Webb — will be over 70 years old on Inauguration Day 2017. Frontrunner Hillary Clinton will be nine months short of 70. Only Martin O'Malley, who will turn 54 a couple of days before the 2017 swearing-in, has not reached retirement age already.

In 2008, Democrats had a 47 year-old candidate who mesmerized the party and ran away with the votes of Americans aged 18 to 29. Republicans, meanwhile, ran a 72 year-old man whose reputation was based on heroism in a war 40 years earlier. Youth won.

This time the situation is reversed. The average age of the Republican field is far below the Democrats, with every candidate younger than Clinton. The most senior is Jeb Bush, who will be 64 on Inauguration Day. Scott Walker will be 49; Marco Rubio will be 45; Ted Cruz, 46; Rand Paul, 54; Chris Christie, 54; Mike Huckabee, 61; Bobby Jindal, 45. Although Bush is in the older range, they're all in the career sweet spot to win the White House.

What accounts for the Democrats' dramatic change from the party of youth to the party of age?

"It's the snuffing out of young talent by the strength and size and sheer velocity of the inevitable nominee," says a well-connected Democratic strategist. "The Clintons took all the air out of the collective Democratic room. There are a lot of people who would be running who are much younger, but they've got their future in front of them, and they don't want the Clintons to ruin it, in this campaign or after this campaign. So they're waiting for a moment when there is enough oxygen to run."

"If Hillary Clinton weren't running, we'd have a field that looks like the Republican field — young and vibrant and diverse."

If there were no Clinton campaign, would Democratic Sen. Kirsten Gillibrand, who will be 50 on Inauguration Day, be exploring a run? Would Sen. Amy Klobuchar, who will be 56, be thinking about it? Would New York Gov. Andrew Cuomo, who will be 59, be preparing to run? They'd all be considering it.

If any of them ran today, however, they could find themselves unable to gain any traction at all in a Clinton-dominated primary race. That could make it more difficult for them to mount a credible run in the future.

In addition, any Democrat running against Hillary Clinton today would face the question of whether to aggressively attack the frontrunner, which, after a losing campaign, could result in dreaded recriminations from the powerful Clinton camp. If, on the other hand, those politicians sit this one out, they might have a chance for a real shot someday.

The non-Clinton Democrats thinking about running today are past all that. "They're just a bunch of old farts who don't have a future," says the party strategist. "They're at the end of their road, not the beginning. Their future is right now, so they're not afraid of the Clintons." (The exception is O'Malley, young enough but now out of office with no other path immediate available.)

The elderly field is dismaying for some Democrats, but not overwhelmingly so. They believe, rightly, that if a Republican wins in 2016, then everything Barack Obama has accomplished in eight years will be endangered. To protect those achievements — most notably, national health care — a Democrat must keep the White House. If that is Hillary Clinton, fine.

It follows that some Democrats are probably rethinking their attitudes toward old candidates. And indeed they are. In a February 2007 Pew survey, a solid majority of Democrats, 60 percent, said they would be less likely to support a presidential candidate in his or her 70s. A year ago, in April 2014, with a Clinton run looming, just 44 percent of Democrats expressed similar reservations about a 70-plus candidate.

If Republicans nominate Bush, the oldest GOP candidate with a family name that has been on national ballots since 1980, it's unlikely age would be an issue in a general election campaign.

But if Scott Walker or Marco Rubio or another relatively young candidate wins the GOP nomination, the voters' views on age could be put to a real test. Rubio has already presented himself as a young man ready for the future. Walker would certainly do the same. They could plausibly frame the race as future versus past.

As for the Democrats, they're stuck with the candidates they have, working to make the best of their new status as the party of times gone by.