It all depends on the individual.
At my local airfield we have a spritely 90 year old who is still flying on his NPPL - I would happily be his passenger anytime.
My father, a veteran instructor, was still examining when he was 81 years and had all his faculties with much sage advice to pass on.
I read quite a few classic stereotypes about "old age" on this thread. Feature on news about pensions a while ago and they show folks walking with Zimmer frames!
Latest research shows dementia and type 2 diabetes go together and can be prevented and even reversed by correct nutrition - read "Grain Brain" by David Perlmutter.
It's a tough call I agree though. Although I have no health issues I decided to quit my last airline job when I was 62 years (I still do some light aircraft flying) since I thought it preferable to do so when "winning"!