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.

[0]: https://2700chess.com/?per-page=100

There's a sharp decline with age. Magnus himself says he's not as sharp as he was younger, even if he can compensate with experience.

For most people there is a cognitive decline with age, and chess is clearly a cognitive effort. Like with everything else: experience really matters, but you will simply be a bit less sharp over time and in a game where a tiny mistake can compound to a loss it really matters.