Joe Biden, America's oldest president, received support from an unexpected ally over his age: Donald Trump said the Democrat was "not too old" to seek a second term, while quickly adding: "I think he's incompetent."

The former president's comments, made in an interview with NBC's "Meet the Press" to be aired Sunday, come as America faces a profound debate over the aging of its political class, a major factor ahead of the 2024 elections — when Biden will be just shy of 82 and Trump himself 78.

Polls show that while the age gap between the two men is small, Americans worry more about Biden's age, with one survey finding that three in four people doubt his ability to last a second term.

That Trump ended his own term in office as the second-oldest president ever (Ronald Reagan was 77 at the end of his term) presents the real estate magnate with a challenge, however, in repeatedly portraying Biden as too "sleepy" to do the job.

But that hasn't stopped the Republican, who would be 82 at the end of a second term, from trying to thread that needle.

"Some of the greatest world leaders have been in their 80s," he told NBC. "I'm not anywhere very near 80, by the way.

"And Biden's not too old," Trump added. "But I think he's incompetent, and that's a bigger problem."

Biden, who overcame a stuttering problem as a youth, has often been mocked for his verbal stumbles.

But he remains trim, physically active, does not drink and is often seen riding his bicycle during weekends at his Delaware home.

Democrats say he also fares well on the competency front, having overseen the passage of major legislation, including a massive infrastructure bill, the largest gun-safety bill in decades, and the Build Back Better social spending program.

Biden has also been active on the global stage. Indeed, during his just-ended trip to Vietnam — which analysts say was staged partly to highlight his vigor — that country's 79-year-old Communist leader smilingly complimented Biden on his relative youth.