WASHINGTON, United States -- 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.

US President Joe Biden turns 81. (AFP)

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.