Joe Biden will turn 81 on Monday with the traditional pardoning of the Thanksgiving turkeys at the White House, but don't expect him to mention the issue that has US voters in a flap.
His birthday will turn an unwelcome spotlight on the fact that the Democrat is the oldest president in American history -- and that if he wins a second term next year he will be 86 by the time he leaves.
Biden sometimes jokes about it and aides point to a vigorous schedule that would floor far younger people, but poll after poll shows that the president's age is the single greatest concern for American voters.
That has been reinforced by a series of trips, slips and stumbles, from losing his balance on the steps of Air Force One to giving occasionally rambling answers during press conferences.
Despite a series of his own recent gaffes, former president Donald Trump, whom Biden is likely to face next year, does not yet cause the same concerns among voters despite the fact that he's 77.
Biden is "not doing a lot wrong" but is struggling to change perceptions on his age -- as well as other issues like the economy -- said David Karol, who teaches government and politics at the University of Maryland.
"He is lucid, but people just have this perception," Karol told AFP.
- 'Competence' -
Biden has increasingly been making light of his age to deflect or defuse people's concerns.
On a visit to meet union workers in Illinois he joked that "I know I only look like I'm 30, but I've been around for a long time" -- and when a reporter tripped in the media area, Biden said, "I just want the press to know that wasn't me."
On other occasions, Biden has used it to portray himself as a safe pair of political hands, saying that age brings with it a little wisdom.
But if re-elected, he would leave office a full nine years older than record-holder Ronald Reagan was when he stepped down at 77.
The White House has been dismissive of opinion polls, with Democrats notching up a series of recent electoral successes.
But the numbers make grim reading for the party. Seventy-four percent of people said Biden would be too old to serve a second term, compared to 50 percent for Trump, a recent ABC/Washington Post poll showed.
A Yahoo/YouGov poll found that 54 percent of Americans say Biden no longer has "the competence to carry out the job of president," up from 41 percent before the 2020 election.
Some analysts say Biden's age should not matter.
The issue in general has been unfairly "weaponized" in US politics, said S. Jay Olshansky, a longevity researcher at the University of Illinois at Chicago.
"Aging is not what it used to be," Olshansky told AFP.
"There are very large segments of the population that survive to their eighth decade perfectly capable of being president, or doing whatever they like."
- 'Super-Agers' -
Biden -- and Trump for that matter -- are both likely to be "Super-Agers," a term coined by researchers to describe a small group of people who keep their full faculties until late in life, said Olshansky.
His research has also found that for US presidents, "biological time seems to tick at a slower pace" than for other people, as they apparently thrive on the stress of the job.
That said, Biden's campaign has taken to highlighting what it says are slip-ups of Trump's own.
Trump warned in a speech in September, for example, that the United States was on the verge of "World War II," and recently said Hungarian Prime Minister Viktor Orban was the leader of Turkey.
Nevertheless, Biden's age is bound to come under even deeper scrutiny during a grueling election campaign.
Republicans have trained their fire on the person who is just a heartbeat away from the presidency, should the worst happen: Vice President Kamala Harris.
Harris blazed a trail as the first woman, Black person and person of South Asian descent to hold the vice president's office, but her approval ratings are as bad as her boss's, at under 40 percent.
77-year-old Trump also under age scrutiny
If either Joe Biden or Donald Trump prevails in the November 2024 US elections, the next president will be the oldest in history to take the oath of office.
But while Biden's age and public slips and stumbles have been more widely scrutinized and examined, his blustery Republican rival's missteps are coming under sharper focus too.
In recent weeks, Trump has confused the Hungarian leader for the Turkish one, claimed that he defeated Barack Obama -- and not Hillary Clinton, his actual rival -- in the 2016 election, and warned that the world might be headed for a second, rather than a third, world war.
Rivals of the former president have been having a field day with such bloopers, quick to remind voters that Trump, the overwhelming favorite in Republican primary polls, is not so young himself.
Videos of the 77-year-old real estate tycoon stammering, verbally stumbling, or looking frail and sweaty have been gleefully posted on social media by the campaign team of Ron DeSantis. The Florida governor, a youthful 45, is currently leading Trump's Republican rivals in opinion polls in most states.
- 'Weak and frail -
Biden's team, meantime, is not unhappy at being able, for once, to turn the spotlight on another aging candidate -- especially as the current president is turning 81 on Monday.
But the Trump camp has not taken such slights lightly.
"The Biden campaign must be confused like their own candidate because Biden is in a constant state of confusion," Trump spokesperson Steven Cheung told AFP.
In an unusually blunt and biting statement, Cheung went on to mock Biden as "weak and frail," prone to fall and even "just plain stupid."
Trump, during his campaign rallies, generally appears vigorous, interacting sometimes for hours with a sea of supporters.
But critics ask whether his repeated gaffes and the videos of him gripping a railing on a ramp to keep from falling, or trembling as he holds a glass of water, are reflections simply of fatigue -- or signs of physical or mental decline.
From the outside, it is impossible to say.
While the two men are not far apart in age, Biden likely comes across as older, said political scientist Kyle Kondik.
The polls tend to agree: Two-thirds of Americans believe the current president is too old to serve a second term, while only half of those surveyed say the same about the slightly younger Trump.
It remains to be seen how the two will hold up once the campaign is fully under way, said Kondik. Rightly or wrongly, he added, Trump's verbal slips seem to be judged more leniently than Biden's, simply because Trump has made so many of them.
- Fast food -
The Republican also does not face the same scrutiny over his physical condition as does his Democratic rival.
As US president, Biden -- who is frequently seen biking with his Secret Service detail -- faces an extensive series of annual medical tests, the results of which are reported in detail to the press.
While Biden's latest health summary described him as "healthy (and) vigorous," it also mentioned that a small lesion had been removed from his skin and that he had undergone a colonoscopy in 2021.
But since Trump left office three years ago, almost no details have been provided about the health of the former president, a known lover of fast food who appears to get little exercise except on the golf course.
The age of presidents was a matter of lively debate during the second term of Ronald Reagan, in the 1980s, when some observers speculated that he seemed to be showing signs of mental decline.
Years after he left office at age 77, it was announced that Reagan was suffering from Alzheimer's disease.
In 1994, former president Jimmy Carter himself sounded an alarm in an article in the Journal of the American Medical Association, warning of the "danger" to the United States if a president suffered a "neurological disease" that reduced his capacity for leadership.
The DeSantis campaign has increasingly been highlighting Trump's age and slip-ups.
"The president is not a job for an 80-year-old," DeSantis said Sunday on CNN.
Nikki Haley, a 51-year-old currently seeking the Republican presidential nomination, has repeatedly called for mandatory mental competency tests for politicians over 75.
But nothing of the kind appears likely anytime soon.