One form of mental exhaustion though that I have, it is entirely non physical in origins, and not easy to discuss.
I grew up with my nose in a text book.
I have mentioned it before in posts here. I think I have over educated myself. I think I have delved into too many places, into too many topics and done it too deeply.
We all worry about the big questions of life, but, I don't think most do it quite as intensely as I have.
Death, do you worry about it much?
Do you really concern yourself about the nature of reality?
Do you wonder much about whether this life is all that there is?
Are you religious, and does being religious matter much to you really?
I have tackled the above from more directions than most are even aware of in the first place.
And at the end of the day, it can be very tiring.
Some answers, you don't really want to have actually.
I spend a lot of my time being intentionally immature, as I have found that in some respects, being serious too much is actually harmful.
I spent most of the last decade on forums discussing may topics that usually ended in arguing. And in the end, the final result is often discussing politics and religion is about the best way to learn to hate. You don't learn much of anything else other than how to really hate people.
I have found anime to be almost the exact opposite. It is mainly an escape from serious. Most anime is silly.
When a person comments 'isn't that meant for kids', I often wonder, if it is safe for kids, why should I have a problem with it?
Kids should be told to not watch the news actually, it's not safe to watch the news.
All I have gained from 40 years of text books, is a problem with having to many answers making it too hard to avoid being too serious especially when I can usually rip apart a badly stated argument, and not needing to get my opinion by copy pasting something via Google

All I have suffered from from watching shows meant for kids, is the ability to laugh at silly things again, and to remember what being a kid felt like.
To be called 'immature' is to be told you are doing it right.