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 ageing 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.

Trump, for his part, has gotten mostly positive marks during annual physicals -- though he has been classed as overweight and does no exercise other than golf. Like Biden, he does not drink.

Trump, too, has had his share of bizarre verbal moments, mangled pronunciations and physical stumbles. But in the NBC interview he said his own age does not concern him, noting that his parents had long lives.

"So genetically, that's a good thing."

His insistence on his own competency was ridiculed during the 2020 presidential campaign when Trump said he had scored an "amazing" result on what he said was a test of mental acuity.

He said he had had to remember, in order, these words: "Person, woman, man, camera, TV."

His proud and repeated recitation of those five words was ridiculed by political rivals and went viral on social media.

Still, in an apparent reference to that test made in the NBC interview, Trump said, "I aced it. I got everything right."