If Biden behaved adequately, then no one would talk about age even if he was 90 years old. But he often looks into the void, goes in an incomprehensible direction and behaves like an old man who is lost. At the same time, Trump is behaving very cheerfully. This difference has been noticeable to everyone in recent years, so part of the voters will not vote For someone, but rather Against the fact that a person with dementia is the president.
In general, it would be good (for American voters) if they had a wider choice, including younger candidates.
With ever passing day, Biden is resembling more and more like a walking corpse. There are conspiracy theories that claim that the original plan from the Democrats was to replace him mid-way through his term
with Kamala Harris. But that would be another disaster, given her radical left-wing agenda. For 2024, IMO it would be again Biden vs Trump. Kamala is still unpalatable for the vast majority of the independent voters. And by then Biden will be 81 years old and Trump would be 78 years old. I sincerely hope that the GOP would rather nominate Ron DeSantis.
It's good that in the United States the power is quite centralized and no madman, even if he became president, can break everything. Kamala Harris is rejected by the average voter not only because she is a leftist, but because she seems frankly dumb - she laughs dumbly at any questions and spouts some kind of nonsense. Even AOC looks like an intellectual against her background.