Never before has a sitting US president faced 80 candles on a birthday cake -- and the milestone that Joe Biden reaches on Sunday has undeniable ramifications as he ponders running again in 2024.

The White House so far has not revealed any celebration plans, focusing instead on preparations for the wedding on Saturday of Biden's granddaughter.

Biden himself jokes about the big 8-0. "I can't even say the age I am going to be," he said on MSNBC. "I can't even get it out of my mouth.

And he brushes off questions about whether he should seek reelection that would put him in power until aged 86, responding with two words: "Watch me."

'Fit for duty'

A year ago, after an extensive medical checkup, doctors said Biden had only a few minor ailments and concluded that he was "fit for duty."

The slender president neither smokes nor drinks, remains physically active and hasn't had any major health concerns since having surgery for two life-threatening brain aneurysms in 1988.

A study published in 2020 by the University of Illinois even classifies him among the "super agers" -- people who, for socio-economic, lifestyle and genetic reasons, live longer than the average.

The researchers gave him a theoretical life expectancy of nearly 97 years.

But the fact remains that Biden now looks his age: His hair is thinner, his walk stiffer.

He retires to his family home in Delaware nearly every weekend, and in some group photos with fellow world leaders, like French President Emmanuel Macron or Canadian Prime Minister Justin Trudeau, the aged US president looks like a patriarch.

Medical reality is not in Biden's favor -- as the years advance, the risk increases of cognitive decline, illness or after-effects of even minor accidents.

Biden's moments of confusion are more frequent, as are his verbal stumbles. Republicans make hay of it with ridiculing memes and tweets.

'Where's Jackie?'

A few weeks back while addressing an audience, Biden sought out a deceased congresswoman as if she were in the room, asking, "Where's Jackie?"

History suggests that a sitting US president concluding a first term will run again.

One example is Ronald Reagan, reelected in 1984 at the age of 73, in spite of debates about his age.

When a president seeks reelection, "it saves the party from an expensive and divisive party primary" which allows it to focus resources on the general election, said Rachel Bitecofer, a Democratic strategist.

Yet in polling, a clear majority of Americans reject the idea of a second Biden term.

A grassroots Democratic group, RootsAction, has launched a "Don't run Joe" campaign arguing that "he has no automatic right to renomination."

Among the open questions are whether a president born during World War II can mobilize young voters in 2024, even if his policies have been favorable to them on issues like marijuana decriminalization and student loan forgiveness.

But if not Biden, who?

Vice President Kamala Harris is not popular, and other prominent Democrats, like Transportation Secretary Pete Buttigieg, may be seen as too green.

"If there's going to be a challenge of a sitting president, usually it is kind of a firebrand... somebody who really wants to push hard," said Robert Rowland, a political communication expert at the University of Kansas.

"The most obvious person to do that would be Bernie Sanders, but (at 81) he's older than Biden."

© Agence France-Presse