> the stress of training and competing at that level must get old after a while.

The stress of elite competition clearly has a shelf life, but Magnus is not overly old. Cognitive performance typically hits a plateau at 35 years old and begins a sustained decline after 45 years old.

The current youth wave of GMs is likely a function of compressed training efficiency. Modern players reach the 10,000 hours threshold much earlier because they had greater access to better training material and had better practice.

The youth wave of GMs is also going to be driven by a general increase in the popularity and image of chess. There's probably way more parents competently teaching their children chess than there have ever been. This may be playing an even bigger role than the training itself. For instance Gukesh's coach was actively running an experiment on him, and as a result he did not use engines in his training until he was already 2500+.