For some concrete numbers, there are only four players over 50 years of age in the top 100 at the moment by live ratings[0]. They are ranked #13 (age 56), #89 (age 53), #95 (age 54), and #97 (age 57). In their primes these players were ranked #1, #10, #4, and #3 respectively.
This is some fascinating data, thanks for pulling it together.
Isn't he playing Chess960 because he started finding standard chess boring? And wasn't that why Fischer worked on it in the first place? Experts might get bored of it by the time they're 50.