US President Joe Biden made light of conspiracy theories about him and pop superstar Taylor Swift during an appearance on a late night show and had a ready answer to the question of whether she'll endorse him in 2024: that's classified.

During an appearance on NBC's "Late Night with Seth Meyers" on Monday, Biden, a Democrat, joked with Meyers, a comedian, about conspiracy theories that the president and the singer-songwriter are in "cahoots." Meyers said recent polling showed 18% of Americans believed Biden and Swift were somehow working together.

"Can you confirm or deny that there is an active conspiracy between you and Ms Swift?" Meyers asked Biden.

"Where are you getting this information?" the president quipped, to laughter. "It’s classified."

Swift, whose massive popularity with young people would be a boon for the president as he runs for re-election in 2024, endorsed Biden in 2020, the president happily pointed out.

"You think it might come around again?" Meyers asked.

"I told you, it’s classified," Biden replied.

Biden's appearance on the show, which was taped on Monday and broadcast early on Tuesday, is the latest attempt by the president to connect with younger people and dispel concerns about his age after a special counsel's report raised questions about his memory and mental acuity.

Biden, 81, noted that former President Donald Trump, his likely Republican rival, 77, was close to him in age and has had his own problems with verbal slip-ups.

"You got to take a look at the other guy. He's about as old as I am, but he can't even remember his wife's name," Biden said, referencing Trump's recent appearance at the conservative CPAC conference in which some thought he referred to his wife, Melania, as "Mercedes."

Mercedes Schlapp, a former communications official in Trump's White House, is one of CPAC's organisers. The former president's reference may have been to her.

Biden also sought to address concerns about his age by comparing his policies with Trump's in areas such as abortion. Biden supports women's rights to abortion; Trump has expressed pride in naming three new justices to the Supreme Court during his tenure who later helped repeal the landmark Roe v. Wade decision that had guaranteed abortion rights nationwide.

"It's about how old your ideas are. Look, I mean, this is a guy who wants to take us back," Biden said, referring to Trump. "He wants to take us back to Roe v Wade. He wants to take us back on a whole range of issues."