The question is, would you replace this person with a younger version of her? The final result in effect being that votes wouldnt be any different? Or would you replace her with someone in your own image? Ask the most vocal voices using this argument and the answer is the latter. Age is just a convenient excuse for many to try and achieve their own partisan goals. Besides you can be younger, and look even younger than your actual years if you have the genes or the money, but still be a brainless and self-absorbed fool that I wouldnt let pump gas into my car.
After all, look at next crop of presidential candidates. The two republicans, the front runner would be almost 80 if he won. The runner up, still in his 40's. Yet, if the primaries were all held today the old man would get the nomination. Of course we have an old man that Democrats will likely chose as their next candidate. Our two main contenders, chosen by both respective parties, old men.
Then again, if you want to take simple numbers out of the equation (for both Trump and Biden), the easy argument used, but focus solely on health and physical + mental fitness how do you do that? And by whom? Full medical workups, with full data made public (HIPA not withstanding), by doctors with no bias or relations with the patient? Not simply 'he's good to go'? Psychologists for mental evaluations, also with data? Imagine if both had to pass extensive psychological exams. My guess, both would have failed. Would have had to be done before the first primary of course so primary voters could made different choices. Interesting to consider who those two would have been.