At least one of the reasons.
Sometimes I have these realizations unrelated to mathematics, let's call the little insights.
They are trivial but yet through constant exposure I had failed to notice them.
I'm 27, not a mathematician but have gone through some difficult mathematics textbooks with ease.
Synapses and whatnot in your brain get pruned mercilessly and you become duller.
This is the little spark that lets you solve problems in minutes instead of hours.
Stamina on the other hand is unaffected.