That's been my experience as well.

I'm much happier in my 30s, having enough experience that I've learned to let go and accept existential dread for the useless bother that it is, than I was in my 20s. Truly realizing that nobody's that special also was a qualitative leap.

Not that I'd call myself « smart » with a straight face, but I certainly used to care a lot about things outside of my control.

Ding! At a certain point constant existential dread becomes tiring. You spend years mulling on it and the answer continues to be there’s nothing you can do, so what’s the point in worrying? Might as well go do something else and enjoy yourself.

Could it be that 'smart' people have accomplishment or material gain later in life they are satisfied with?

I've definitely gotten better at consciously adding things the list of shit I try not to to think about. Which is pretty long at this point.

I finally tried weed well into my 30s and discovered it's great at making sure I don't think about those things when I'm trying to go to sleep, which is really nice.

In general I'm pretty sure I've got a fair amount of derealization going on all the time now, which... helps?

I suppose that all adds up to some kind of mellowing out.

Yeah, I find that looking at anything space-related helps me remember that our entire species and history are completely and utterly insignificant in the grander scheme of things. All this day-to-day drama just really isn’t that important.

Cognitive decline is a natural consequence of aging

A possible explanation is that the mellowing is due to the loss of acuity with age. The more likely explanation is that it's an evolutionary adaption in response to how ones function in society changes with age.