Washington (AFP) - With a septuagenarian president preparing his re-election bid, and two likely main challengers in their late seventies by 2020, Americans are facing an inevitable question: does age matter in White House politics?

Donald Trump, the oldest person ever elected US president, will be 74 on Inauguration Day 2021.

Two likely contenders for the Democratic nomination who lead early polls, Senator Bernie Sanders and former vice president Joe Biden, will be 79 and 78, respectively. Senator Elizabeth Warren, the Massachusetts progressive also in the race, will be 71.

Democrats already sport several far younger White House hopefuls: Tulsi Gabbard and Pete Buttigieg are 37, Julian Castro is 44 and Senator Cory Booker is 49.

History would suggest they have a better shot at victory than members of the gerontocracy. The last time Americans elected a Democrat older than 55 to the White House was Harry Truman in 1948.

As Americans mull the prospect of a president turning 80 in his first term, two images from recent days stand out.

Candidate Beto O'Rourke, 46, drew attention to his own comparative youth, running a five-kilometer (three-mile) race at a brisk clip Saturday in Iowa, while Bernie Sanders, 31 years his senior, slipped in the shower and sustained a gash on his head that required seven stitches.

(Sanders has been given a clean bill of health.)

For Biden and Sanders, their decades in the political arena mean greater name recognition among voters and valuable experience that they can apply to the most powerful position on Earth.

It could also weigh them down, as they seek to expand support and embrace modern forms of communication like social media.

Biden and Sanders backers frequently brush off questions about their political favorites' ages, touting their vitality and firing off ageism accusations.

Sanders can appear to have more support from young people than senior citizens.

"His age doesn't matter to me, (and) there's no reason to believe he's not healthy," Garrick Dodson, who is 17 but will be old enough to vote next year, told AFP in Iowa.

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He said Sanders inspired "a whole generation of kids" in the 2016 presidential race, with his progressive platform that includes universal health care and the fight against wealth inequality.

"I love Bernie Sanders and will vote for him no matter what," Dodson added.

Trump, who delights in confounding health experts with his junk food cravings, was declared to be in "very good health" last month by his chief physician.

"And I anticipate he will remain so for the duration of his presidency, and beyond," chief physician Sean Conley said in a statement.

But a rambling series of bombastic tweets by Trump in recent days caused concern.

Aides defended the president's mental state. But lawyer George Conway, the husband of Trump counselor Kellyanne Conway and a regular critic of the president, insisted Trump's "condition is getting worse."

- 'Harder' at 80 -

Age has been no barrier to success in US politics.

House Speaker Nancy Pelosi has shown that, at nearly 79, she remains a remarkably shrewd politician. Senate Majority Leader Mitch McConnell, 77, is also fully engaged and a master tactician.

Trump infuriated Democrats in 2016 by attacking Hillary Clinton, then 68, with claims his rival lacked the "mental and physical stamina" to be president.

Sanders, who would be the oldest-ever president by far, is firm on the issue: Voters need to treat candidates "not by their sexual orientation or their gender, and not by their age," he said last month.

"I have been very blessed in my life with good health... and I think I had and still have a great deal of energy."

But gradual decline is inevitable. Eric De Jonge, director of geriatrics at MedStar Washington Hospital Center, said aging's effects become particularly noticeable after 80, when dementia becomes more prevalent.

Organs naturally degenerate, there is neuron loss in the brain, blood vessels stiffen, exercise capacity decreases.

"Exhausting 16-, 18-hour campaign days are going to be harder on an 80-year-old than on a 50-year-old, all other things being equal," De Jonge told AFP.

"That doesn't mean you can't have a great candidate who's 75 years old."

For those entering the presidency in their late seventies, physical and cognitive deterioration could become an issue in their second term.

De Jonge believes age is likely to play out in the 2020 campaign.

"People will react to an incredibly robust and energetic 75-year-old and think 'Wow, that guy can really handle it and keep up with everybody, good for him,'" he added.

"Or it may play out in the other way, that they may reveal certain symptoms of aging that are of concern to the electorate."