Washington: Joe Biden has joked about it, ignored it and suggested it makes him wise. A year out from the US presidential election, the issue of his age just won't go away.
The 80-year-old Democrat is already the oldest president in American history, and if he wins a second term next year he will be 86 when he leaves the White House.
A series of trips - most famously stumbling on the steps of Air Force One - and verbal slips have gone viral on social media and given Republicans fodder for attacks.
Polls show that it's a major worry for US voters concerned about what would happen if their commander-in-chief became incapacitated, and that it is distracting from his efforts to sell his policies.
Worse still for Biden, voters don't seem to have the same concerns about Donald Trump, despite the fact the former president and Biden's likely Republican rival is aged 77.
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.
But age in general has been unfairly "weaponized" in US politics, says S. Jay Olshansky, a longevity researcher at the University of Illinois at Chicago.
"Ageing is not what it used to be," Olshansky told AFP.
'Wisdom'
"There are very large segments of the population that survive to their eighth decade perfectly capable of being president or doing whatever they like," added Olshansky.
"To the contrary, accompanying chronological age is wisdom, knowledge, and experience."
Nevertheless, Biden's age will come under even deeper scrutiny during a grueling election campaign.
Biden must convince people he is fit by hitting the campaign trail "at least five days each week" next year, the Brookings Institute said in a commentary.
Biden's last medical in February described him as "vigorous" but his gait has become more shuffling and his voice often descends into a whisper.
Accidents such as tumbling off his bike have been broadcast worldwide, and he now uses the shorter stairs of the presidential plane to avoid further falls.
His propensity for gaffes - from rambling answers about John Wayne films to saying he wanted to go to bed during a press conference - have also caused confusion.
But Biden, who has suffered from a lifelong stutter, has increasingly been hitting back.
He's joked several times recently about being 800 years old. And when he stumbled while climbing onstage in Philadelphia recently he earned a laugh from the audience when he jokingly clutched the railings for several seconds.
The Israel-Hamas conflict and Ukraine war have also allowed Biden to proclaim the benefits of someone with half a century of foreign policy experience.
'Superagers'
Meanwhile Trump, who has made grotesque impressions of Biden during recent rallies, has been slip-ups of his own.
Trump warned in a speech in September that the United States was on the verge of "World War II" - shortly after he called Biden "cognitively impaired."
The Biden campaign recently sent out an email listing further Trump mix-ups, such as saying that Hungarian Prime Minister Viktor Orban was the leader of Turkey.
In any case, both Biden and Trump are likely to be "Superagers", a term coined by researchers for a small group of people who keep their faculties until later in life, says Olshansky.
Olshansky's 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.
But a key Republican line of attack remains aimed at 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.
A Yahoo/YouGov survey from May showed that only 35 percent of people believed Harris would be ready to step in as president.
In the end, American voters may decide that age is just a number.
"I've never even considered the age thing," said Olivia Besgrove, a 23-year-old nurse from Missouri who leans to the Republicans.
"I just would appreciate someone who does have their head on straight."
