I think there is a lot ot be said for the sheer physical pain of playing and how this will impact upon the decisions older musicians make regarding their career paths. Having read most of his books, I am always interested to hear what he says about drumming, technique etc and comparing his approach to the work of other drummers who play into later life such as Buddy Rich or Art Blakey. On of the Peart books references the pain he gets in his hands, arms and legs and I recall mention of blisters etc. He obviously plays hard which will have an impact upon his body, particularly as he gets older and the healing process is slower (imagine STARTING a gig with a blister). I do gigs on double bass about once a week and am still quite sore afterwards, hands and fingers. Thankfully, it calsm down overnight mostly and I don't need to do any playing until the next gig a week or even two weeks later. If I had to do several gigs a week, I would be in trouble. If Peart is playing hard night after night, week after week, it's probably gonna hurt.
I actually think the obvious reponse would be for them to play differently, to find a way of expressing themselves that doesn't require them to be faux-teenagers 'rocking out'. You don't need to play that hard to play that music, you just need to play smarter. And as for his level of playing excellence. He is arguably (one of) the worlds greatest rock drummer(s). That is, however, being damned with faint praise. The one thing that his books do tell us time and time again is that Peart doesn't practice/play at all for months and sometimes years at a time. That is going to impact upon his stamina and muscle tone etc.
Interesting stuff.