Mid 30s, also have kid and sleep loss but not as bad as before.

I actually noticed serious mental decline when I was burned out in my late 20s. There were real physical symptoms like not being able to look at a text editor for more than 2 minutes. Post recovery of that, I actually feel like my brain recovered a lot once I started learning languages very seriously (mandarin and japanese), starting a few years ago. Brain feels healthy now but I'm acutely aware of where it's not as sharp as before. Playing around with this felt a little like when my brain is trying to build a new grammar dictionary.