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.