Perhaps I can inject a bit of hard science into this debate.
Correlation is not causation. In other words, just because you see a link between A and B does not mean that A causes B, or that B causes A.
There is a correlation between increasing age and physical deterioration and disability. However, age alone doesn't cause these.
The correlation exists for aircraft as well as people. The older an aircraft is, the more likely it is to fail. However, no regulation that I'm aware of requires aircraft to be scrapped beyond a certain chronological age. Should all DC-3s be declared un-airworthy because of their age? It's exactly the same question that exists for aging pilots.
Currently, DC-3s can continue to fly as long as they are certified airworthy, and age doesn't enter into it. Is there a reason why this should not also be the case for pilots? If a pilot passes all the tests, why should he be forcibly retired just because of age? If the tests don't prove him competent, then the tests need to change.
If you start assuming that age is synonymous with incompetence, then should you extend this to other correlations that haven't been proven to be causation? For example, black people have a much higher incidence of severe hypertension than white people. Should black pilots be denied medicals because their race is correlated with higher incidence of hypertension? Or should black pilots simply be tested using the same criteria as white pilots, and then be issued medical certifications if they pass the tests, irrespective of race?
Women tolerate high accelerations much better than men. Should men be denied medical certification for flying fighter airplanes because their sex is correlated with a lower tolerance for high Gs? Or should both men and women be tested using the same criteria, with anyone passing the tests being allowed to become a pilot?
Be careful what you wish for.