Joe Biden joked about his age as he turned 81 on Monday, but the issue is no laughing matter for many voters who are worried he is too old for reelection next year.

"By the way it's my birthday today... It's difficult turning 60," the president said with a chuckle at the annual Thanksgiving turkey pardoning ceremony at the White House.

In front of an audience including schoolchildren, he then quipped: "This is the 76th anniversary of this event -- and I want you to know I wasn't there for the first one."

But a moment in which he mixed up US singers Taylor Swift and Britney Spears soon baffled his guests -- and renewed focus on his age.

The Democrat is the oldest president in American history, and if he wins a second term next year he will be 86 by the time he leaves.

Poll after poll shows that a majority of voters think he is too old to be commander-in-chief. Incidents in which he has tripped, including on the stairs of Air Force One, repeatedly go viral.

Voters don't so far have the same concerns about his likely election rival Donald Trump, despite the fact that the Republican is 77 and has been known to make similar slip-ups.

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.

- 'Wisdom' -

 

"It's not about age. It's about the president's experience," White House Press Secretary Karine Jean-Pierre said Monday.

She also hailed Biden's "wisdom" and "stamina," reminding reporters of his grueling, secret trip to war-torn Kyiv earlier this year.

Biden will celebrate his birthday as he normally does with his family during their annual visit to the island of Nantucket this week -- "with a coconut cake, which is something that they traditionally do," Jean-Pierre added.

The White House has been dismissive of opinion polls, with Democrats notching up a series of recent electoral successes.

Yet 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.

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.

The issue in general has been unfairly "weaponized" in US politics, added S. Jay Olshansky, a longevity researcher at the University of Illinois at Chicago.

"Aging is not what it used to be," Olshansky told AFP.

- 'Super-Agers' -

 

Biden and Trump 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 Trump's own slipups ahead of a possible 2024 rematch.

Eager to hit back, Trump released a doctor's letter on Monday referencing an examination two months ago in which tests showed his "cognitive exams were exceptional."

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.

The vice president was one of the first to offer her best wishes to Biden on his birthday on social media, saying she was "proud to be in the fight alongside you."

First Lady Jill Biden followed with a simple message, adorned with a hearts emoji: "Happy Birthday, Joe! I love you."