Things related to age are probabilities, not set facts. You may get an 80 year old who is switched on but the chance that a mentally there 80 year old sliding into being senile over 4 years is vastly higher than for a younger person, so even if they seem fine at the start, there is no guarantee it will end that way. Biden and Trump have both been showing signs of losing their minds to varying degrees for a while and yet they are still being selected. My Gran went from being fully aware, if a bit prone to dozing off, to unable to recognise her own family in under a year.