Speaking from experience, being an old, retired, past-it ATCO, there are two competency curves here. There is the "young brain lightning speed of thought" slowly descending curve, and the "years of experience" ascending curve. Yes, the thinking processes do slow down as you age BUT this is compensated for by the growing bag of experience.
However, there comes a point where experience ceases to make up for the speed of thought (or lack of it), probably where the curves cross, and that is the point to say "OK, I quit". It varies from one individual to another, and it takes a degree of personal honesty to admit it to yourself.
Frunobulax
I haven't seen it all.
In my experience you have never seen it all! Experience is what you get just after you needed it.