It's a perfectly valid question and one which is currently hanging over much of the media coverage of the early stages of the 2024 US presidential election. The US president is, after all, currently eighty-one-years old, making him the oldest US president ever, ahead of previous champion, Ronald Reagan who was 77 when he left office in January 1989. President Joe Biden is running for re-election and would be eighty-six by the time a second Biden term of office would end in 2029.
Author Earl Hutchinson summarises the recent media discussion about Biden's health in this short book. For context, he compares the situation now to Reagan's campaign for re-election in 1984 as well as to the septuagenarian Senator John McCain's unsuccessful campaign for the White House in 2008 although curiously Bob Dole's similar 1996 attempt to unseat President Bill Clinton in 1996 (which also occurred when the candidate was 73) is not mentioned. The relative old age of other senior public officials such as Mitch McConnell, Nancy Pelosi and, of course, Donald Trump himself and the possibility of implementing an age limit is also discussed.
In truth, there seems little real reason on current evidence to doubt Biden's ability to lead at all. A more pressing concern is the danger that a growing perception that he is too old could harm his chances of re-election, opening the door to the horrifying prospect that the disgraced, twice impeached and once defeated former President Donald Trump might succeed in winning the White House again in November 2024.