As President Biden celebrated his 81st birthday Monday, some critics doubted he would be able to complete a second term if re-elected amid lingering doubts about his cognitive faculties.

The White House also reportedly downplayed the commander-in-chief's big day. It was a noted departure from former President Barack Obama's 50th birthday bash, ex-President Ronald Reagan's 70th, and the 1948 installation of the White House bowling alley as a gift for then-President Harry Truman's 64th.

Marc Thiessen, a former speechwriter for former President George W. Bush, told FOX News that Democrats are beginning to join the chorus of Republicans who have been pointing to Biden's age as a major concern moving forward.

"If you take Joe Biden off the ticket, [recent polls predict] the Democrats crush Donald Trump in all the swing states... So the answer for the Democrats is to replace Biden because everybody knows he is not going to be able to complete a second term,"he said.

NO LABELS LEADER: JOE MANCHIN COULD GET SERIOUS 2024 CONSIDERATION

"There is no way that he is going to be president five years from now."

Thiessen added that some of the president's critics may have been more subdued in their doubts if the economy was humming along and the U.S. wasn't at least indirectly involved in two wars.

He said one potential alternative to the Democratic Party quietly showing Biden the door would be to replace Vice President Kamala Harris with a Democrat viewed as more competent, and imply that the president has chosen his de facto successor.

The problem, he underlined, would be the left's adherence to political litmus tests.

"They'll never fire the first Black woman vice president. So they're trapped by their own identity politics," he said.

New York Times reporter Peter Baker said on MSNBC the president won't do "anything in public" to celebrate and "does not want to call attention to" his age, adding the White House is likely tired of the subject.

IMPEACHMENT THE BEST PROCESS TO HOLD BIDEN, MAYORKAS ACCOUNTABLE: MILLER

"[B]ut it is one of the most salient issues out there, according to all the polls," Baker said, adding that it is more than just "Trump people" speaking out these days.

Mark Penn, a former pollster for ex-President Bill Clinton, further told "The Story" he's been sounding the alarm on the topic for quite some time.

He added however, that the youth vote currently splintering from Biden is not likely to end up in Trump's column, should he be the GOP nominee.

CLICK TO GET THE FOX NEWS APP

"But I do think the president's got to act on these numbers," he said of a recent poll showing Biden trailing Trump in five of six swing states.

At present, a handful of incumbent lawmakers are still older than Biden, though his critics often claim the issue is one of age combined with faculties.

The oldest sitting senator is currently Republican Chuck Grassley of Iowa at age 90, followed by Sens. Bernie Sanders, I-Vt., at 82 and Senate Minority Leader Mitch McConnell, R-Ky., at 81. 

Reps. Grace Napolitano, D-Calif., and Bill Pascrell Jr., D-N.J., are the oldest House members at 86 years of age.