The former president’s statement comes at a time when there have been calls for ageing politicians, such as Mitch McConnell and 90-year-old Dianne Feinstein, to retire from public offices.
Mr Biden, who will turn 81 in November, is just three years older than Mr Trump, but there has been concern about his ability to continue as a president. Mr Biden is also eyeing a re-election in 2024, where he could go against Mr Trump for a second time.
“CROOKED JOE BIDEN is NOT too OLD, he is too INCOMPETENT!” Mr Trump, 77, wrote in a Truth Social post.
A recent poll by The Wall Street Journal found that 73 per cent of Americans thought Mr Biden was “too old” to run for re-election, while 47 per cent held the same opinion about Mr Trump.
Nearly 60 per cent of the polled responders said that the sitting president was not “mentally up for the job of president” compared to 49 per cent who held the same view about Mr Trump.
Earlier in May, Mr Biden laughed off concerns about his age, saying: “The only thing I bring to this career after my age – as you can see how old I am – but is a little bit of wisdom.”
“I come to the job with more experience than any president in American history. It doesn’t make me better or worse, but it gives me few excuses.”
Meanwhile, attorneys for Mr Trump have filed a motion asking that federal district court judge Tanya Chutkan recuse herself from his 6 January criminal case.
The motion states that judge Chutkan has said in connection with other cases that Mr Trump should be prosecuted and imprisoned and argues that this disqualifies her from the case.
The attorneys asked the federal judge, who is presiding over his election subversion case in Washington, to recuse herself, saying her past public statements about the former president call into question whether she can be fair.
Judge Chutkan is a former assistant public defender who was nominated to the bench by president Barack Obama. She will rule on the motion herself.