Biden turns 81 as questions about age continue to dominate 2024 race

President Joe Biden is celebrating his 81st birthday on Monday, as questions about age continue to dominate the 2024 election.

Biden is the oldest commander in chief in U.S. history. If reelected, he will be 82 at the start of his second term and 86 when it ends.

Former President Donald Trump, the Republican front-runner, isn't far behind at 77 years old. But Biden has so far faced more scrutiny over his age than Trump, though both have experienced noticeable blunders on the campaign trail.

Biden's common refrain to those who press him on his age has been to "watch me." The administration defends his stamina and underscores his record when questioned about his ability to do the job, with his stiff gait and verbal gaffes regularly seized on by critics and Republicans.

“You know, as they say, the proof is in the pudding, right?" White House press secretary Karine Jean-Pierre said Monday. "The president has used his experience to pass more bipartisan legislation in recent times than any other president. That’s just a fact. That’s something we have seen this president do and that’s because of his experience. He's been able to manage multiple foreign policy challenges, that’s -- he’s been able to do that because of his experience."

But poll after poll shows the issue is still top of mind for voters.

Three-quarters of Americans (74%) said Biden was too old to run for another term in a survey conducted by ABC News and the Washington Post in September. That number represented a 6% increase since May, a month after Biden launched his second White House bid.

Half of Americans (50%) thought the same of Trump, according to the poll.

A CNN survey out of New Hampshire -- which will hold the first presidential primary in January -- found 56% of likely Democratic primary voters said age was their biggest concern regarding Biden. The poll, conducted by the University of New Hampshire, was released last week.

Biden frequently jokes about his age on the campaign trail.

"I just want you to know it's difficult turning 60. Difficult," he joked on Monday as he participated in the annual Thanksgiving turkey pardon at the White House.

"This is the 76th anniversary of this event. I want you to know I wasn’t there for the first one," he quipped. "I was too young to make it up."

Questions about age and politics are not limited to Biden. This year, alarming episodes involving longtime Republican Senate leader Mitch McConnell and powerhouse Democrat Dianne Feinstein, who died in September, made headlines and stoked fierce debate about how old is too old to serve in public office.

S. Jay Olshansky, a professor of public health at the University of Illinois at Chicago, previously told ABC News such a question is "unanswerable."

Last year, Biden marked his milestone 80th birthday with a subdued brunch at the White House with his family. At the time, Biden was mulling whether to run again as other veteran Democrats like former House Speaker Nancy Pelosi were announcing they were stepping down from their leadership roles to usher in a new generation.

Jean-Pierre said Biden will celebrate his 81st birthday on Tuesday when in Nantucket with his family and some coconut cake.

-ABC News' Molly Nagle contributed to this report.