

This was from online studies, so take it with a grain of salt, but from https://news.mit.edu/2015/brain-peaks-at-different-ages-0306 :
- raw speed in processing information appears to peak around age 18 or 19, then immediately starts to decline.
- short-term memory continues to improve until around age 25, when it levels off and then begins to drop around age 35.
- the ability to evaluate other people’s emotional states, the peak occurred much later, in the 40s or 50s.
Mostly, I would guess what you’re feeling is the grind of full time work. I remember hearing that mathematicians peaked by 30, but a quick search suggests that was always a bias for rewarding people early in their careers and is not backed up my more recent data.





I gather it is not useful to you, but it is exactly the sort of thing I would want to hear, specifically: I would want a link to some data before any random opinion/advice. I could guess and say “lots of us feel the same way”, but I don’t know if that is true – my feeling is not data, and when people try to console me in that manner, it sounds like BS because I recognize they are offering empty platitudes without actually considering my situation. If instead someone can show me they took the time to think about my issue, and have actual information for me, then I am reassured that their opinion isn’t a flippant and unconsidered reflex.