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 for 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 recent gaffes, former president Donald Trump, whom Biden is likely to face next year, does not yet cause the same concerns among voters even though 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.