I saw him live in Montreal about 3 years ago and he played non-stop impeccably for 3 hours, dancing around stage, switching instruments, no break. I had to take a bathroom break. I'm pretty sure he's not faltering mentally yet…
One thing I've noticed about older people, and even myself (nowhere near 80).
They have much more uneven cognitive performance. There's just days where their thinking is crap.
I'm 43. I'm nearly as sharp as my peak as I was 20 years ago (and I have a lot more knowledge). But I can only access that "peak" on maybe a third of days now.