I think any arbitrary age limit is wrong, and setting an age so as to keep younger pilots' opportunities moving smartly is wrong on several counts.
The pertinent questions are:
Can he pass the physical?
Can he pass the check ride?
I've geared my life, plans, hopes and finances around retiring at 60 for so long, the thought of remaining any time beyond that is not appealing. I'll be gone. But I defend those who wish to stick around as long as they are able and willing.
We have plenty of external means of determining when it's mandatory to hang it up, such as the check airman and flight doc. Could be time at 60, could be 80, could be 40.