Late-night host Stephen Colbert made a surprising Grim Reaper joke about Joe Biden on Monday as he marked the president's 81st birthday.

Reacting to reports that some close to Biden think he shouldn't be put in "Bubble Wrap" but rather make jokes about his age and not shy away from the issue, the CBS host donned a pair of aviator sunglasses.

"Hey everybody, knock, knock?" Colbert as Biden asked. "Who's there?" the "Late Show" crowd replied.

"Not sure, but he's been standing silently in my doorway for a while now. He's a pale fellow, big cloak, long sharp knife on a pole," Colbert said, as some in the crowd sounded surprised. "Smiling right at me, great set of chompers. Look into his eye sockets and see a little movie about all the fun stuff I did when I was a kid."

JON LOVITZ TEARS INTO COLBERT, KIMMEL FOR PUSHING ‘POLITICAL AGENDA’ IN LATE NIGHT: THEY ‘HAMMER IT TO DEATH’

Colbert also quipped Biden was running around town with poll numbers "less than half his age," in reference to a recent survey showing him at just a 40-percent approval rating.

Colbert

Stephen Colbert imitates President Biden on "The Late Show" on November 20, 2023. (The Late Show / YouTube)

Biden's birthday received perhaps more attention than usual given he's already the oldest U.S. president in history, and recent polling and media reports have reflected both public unease and Democratic Party concerns about his viability as a candidate.

On Monday, New York Times reporter Peter Baker noted on MSNBC that Biden was specifically avoiding a lavish public celebration of the day to not draw more attention to the occasion.

"You’re not going to see a big lavish celebration the way Barack Obama celebrated his 50th birthday in office or Bill Clinton celebrated his birthdays in office with fundraisers and concerts and all that," Baker said. "You’re going to see basically almost nothing."

"I mean, he’s not going to do anything in public about it today, doesn’t want to call attention to it," he added, noting he'll celebrate in private in Nantucket with his family. 

BIDEN SKIPPING PUBLIC BIRTHDAY CELEBRATION TO NOT CALL ATTENTION TO AGE, NY TIMES REPORTER SAYS

Even though the president joked about his age on Monday as he pardoned two turkeys named Liberty and Bell, stating, "I just want you to know it’s difficult turning 60," Baker argued the current administration wants to take the focus away from his age as he runs for a second term. 

Politico also reported some aides want Biden to wear more comfortable shoes and walk less on camera to avoid showing off a stiff gait.

Trump, Biden

An NBC poll released on Sunday showed Donald Trump ahead of President Biden in a 2024 general election match-up. Biden continues to face concerns about his age from the public as well. (Left:  (Photo by Michael M. Santiago/Getty Images), Right: (Photo by BRENDAN SMIALOWSKI/AFP via Getty Images))

Biden's staffers are growing increasingly irritated with stories and questions about his age, considering it D.C. parlor talk that's irrelevant to his job performance. They also note his current most likely 2024 opponent, former President Trump, is no spring chicken himself at 77.

Biden's birthday was also fodder for jokes on other late-night shows on Monday.

NBC "Late Show" host Seth Meyers said Biden had turned 81 in spite of his aides "specifically advising him not to."

CLICK HERE TO GET THE FOX NEWS APP

Fox News' Kendall Tietz contributed to this report.