I agree. I met Knuth briefly after a guest lecture at my university a few years ago and although you could tell his body was getting old, his mind was incredibly fresh.
Although I'm not as bright as him, I can only hope to be as intellectually curious as him at that age.
I don't even think this is controversial, but I don't think it's at all without causation: not remaining curious, keeping the mind stimulated, etc., accelerates one's decline.
If you work in something labour intensive, you should retire young while your body's in good health; if you work in academia you should (strive for emeritus and) never leave! (And if you work in SWE, I don't know, we should probably retire, but then spend more time on our own projects/experiments/reading HN.) (All assuming for sake of argument we're optimising for longevity without considering time with family, having the funds to retire, etc.)
