I'm in agreement with other posters here that it's up to the individual, and their state of health, age seems to have nothing to do with it.
Here's one to ponder - People who have lived for most of their lives at high altitudes, e.g. Nepal, the Andes etc., function just as well at high altitude as do those peolple who have spent their lives at low altitudes. This is due to long term adaption and acclimatisation. It could be argued that the longer a pilot has spent at high altitude, the better adapted and acclimatised older pilot may well handle the reduced atmospheric pressure better than a younger and less adapted and acclimatised colleague!
Regards,
OLD Smokey