On average players start declining in their mid to late thirties, just about the age of Magnus (and Hikaru). But even with that decline, it's not like they simply can't play anymore. Drag Kasparov out of retirement and he's still going to be an extremely strong player, even in his 60s.

And a lot probably comes with environmental rather than physical issues. Staying at the highest level in chess requires never-ending opening preparation and study. This same is about the time that kings of the game have made their dominance clear to the point that there's just nothing more to achieve, start having families, and so on. It's going to be very difficult to maintain motivation.

The rise of freestyle chess could viably see players extending their dominance for much longer, because there's currently believed to be no realistic way to do impactful opening prep in that game.

Vishy's still a top player at 56