Last time I played Squash I thrashed a lad half my age, he was a pilot and very fit but he was just slow to react, (I'm just an average player - nothing special).
Pausing here for MAS, Skol etc. to wheel out the usual round of excuses.
The fact remains that there can be no general rule, age sixty five is a reasonable maximum, after that it should be on a case by case basis after appropriate tests.