My dad around 1993 designed a cipher better than RC4 (I know it's not a high mark now, but it kinda was then) at the time, which passed audit by a relevant service.
My dad around 2003 still was intelligent enough, he'd explain me and my sister some interesting mathematical problems and notice similarities to them and interesting things in real life.
My dad around 2005 was promoted to a management position and was already becoming kinda dumber.
My dad around 2010 was a fucking idiot, you'd think he's mentally impaired.
My dad around 2015 apparently went to a fortuneteller to "heal me from autism".
So yeah. I think it's a bit similar to what happens to elderly people when they retire. Everything should be trained, and also real tasks give you feeling of life, giving orders and going to endless could-be-an-email meetings makes you both dumb and depressed.