WASHINGTON — President Biden turned 81 on Monday — extending his record as the oldest-ever commander-in-chief on the heels of polling showing that most voters think he’s too old for the job as he seeks a second term.

Biden would leave office at age 86 if he wins re-election and serves all four years through January 2029.

“It’s my birthday today … I just want you to know, it’s difficult turning 60 — it’s difficult,” Biden joked Monday morning at the annual pre-Thanksgiving turkey pardon at the White House.

He returned to the subject of his age later, saying “I was too young” to attend the first turkey pardon, overseen by then-President Harry S. Truman 76 years ago.

Biden’s attempt to make light of his age hit a snag moments later when he badly mangled a joke by mixing up pop superstars Britney Spears and Taylor Swift.

The event with turkeys named Liberty and Bell was Biden’s only public event of the day ahead of a family vacation to Nantucket starting Tuesday.

Biden aides are not at all concerned about his seniority, White House press secretary Karine Jean-Pierre insisted at her regular briefing Monday — when asked about alarms raised by David Axelrod, former chief strategist to President Barack Obama.

“There is no alarm happening behind the scenes and I am certainly not going to everybody who has something to say,” Jean-Pierre said.

When pressed about polling on age concerns and Biden’s job performance, Jean-Pierre conceded that the findings on public sentiment may be accurate before adding that the narrative could evolve. 

“I’m not challenging the accuracy here. We are not going to change the minds of Americans, we get that. Americans are going to feel how they feel and we’re going to respect that,” the press secretary said.

“We believe experience — this president having experience to get things done — is important,” she added. “And we know it’s going to take some time for Americans to feel what we have been able to do in this administration.”

In recent public appearances, Biden has sought to laugh off questions about his age by remarking that he’s hundreds of years old.

On one occasion, he even walked robotically around a stage to mock clips circulated by critics of him appearing to be confused leaving his podium.

But polling consistently shows age is a major liability ahead of next year’s election — as much as, if not more than, concerns about Biden’s handling of the economy and other issues.

A New York Times poll this month found 71% of swing-state voters say Biden is “too old to be an effective president,” while just 39% said the same of former President Donald Trump, 77, who is seeking a rematch against Biden next year.

A Wall Street Journal poll released in September found that 73% of registered voters believed Biden was too old, versus 47% who said the same of Trump.

And a Washington Post-ABC News poll in June found that just 32% of voters believe Biden has the mental sharpness needed to be president — while 54% said the same of Trump.

Biden hasn’t helped impressions of his age by falling several times in public while in office. He has also mistakenly referred to Kamala Harris, his vice president, as the “president” at least seven times and told a false story about a deceased Amtrak conductor at least 12 times since taking office.

Rep. Dean Phillips (D-Minn.) launched a long-shot primary challenge against Biden last month, warning his fellow party members that “it looks like on our current course, the Democrats will lose and Trump will be our president again.”

Axelrod, the former Obama strategist, said last week on CNN that “in front of the camera, what he’s projecting is causing people concerns, and that is worrisome.”

Over the weekend, Axelrod told New York Times columnist Maureen Dowd that he put Biden’s chances of winning re-election at “a 50-50 shot here, but no better than that, maybe a little worse.”

“He thinks he can cheat nature here and it’s really risky,” said Axelrod, adding that the White House has “got a real problem if they’re counting on Trump to win it for them. I remember Hillary [Clinton] doing that [in 2016], too.”