I think the degradation with age as in part bio/physiological might also be in part a matter of adjustment to circumstance. That is to say the really hard probl

I think the degradation with age as in part bio/physiological might also be in part a matter of adjustment to circumstance. That is to say the really hard problems in mathematics (or physics) one generally doesn't get to work on full-time (since it's very likely that one will work on them for years without producing any results); the most likely path in academia is therefore to do mediocre -to fairly good research that produces results and publications. When one compromises in to doing work that's less difficult than one can be doing because it's what's necessary career wise, it's not surprising that one's mental alacrity declines. There are certainly exceptional people that can do one kind of thing for work (or tenure), and at the same time publish significant work in a parallel field - but I think it's rare.