Joe Biden, America's oldest-ever president, said Monday he understood a focus on his age but that he was running for re-election because Donald Trump wanted to "destroy" US democracy.

The 80-year-old usually avoids the age issue but addressed it during a fundraiser at a Broadway theatre in New York, saying his experience helped him deal with crises like Ukraine and Covid.

"A lot of people seem focused on my age," Biden said. "I get it believe me, I know it more than anyone."

He added: "I'm running because democracy is at stake because in 2024 democracy is on the ballot once again. And let there be no question: Donald Trump and his MAGA Republicans are determined to destroy American democracy."

The Democrat added that he would not "bow down" to "dictators" and accused Trump — whose slogan is Make America Great Again (MAGA) — of doing so to Russian President Vladimir Putin.

Opinion polls show American voters have concerns about Biden's age ahead of a likely rematch next year against Trump, whom he beat in 2020.

An influential US columnist, David Ignatius of the Washington Post, caused a stir when he called on Biden last week not to run, saying Biden risked undermining his "greatest achievement" in beating Trump.

Biden, who is attending the UN General Assembly in New York this week, would be 86 at the end of a second term and his Republican opponents relentlessly target the issue.

Trump — who is 77 and would be the oldest president ever elected if he wins next year — said in an interview broadcast Sunday that Biden was "not too old" but was "incompetent".