Any average guys who “became intelligent” after 30? Is it possible?

r/

I [29m] am about to turn 30 looking at myself now I straight up I feel like a dumbass and only seem to be getting dumber and slower with age. My brain isn’t nearly as sharp as it was when I was 18 or 24 even. My attention span gets smaller and smaller. It takes me a long time to learn complicated things and I struggle with remembering them.
I wanted to do a masters or post graduate degrees in law or computer science but honestly I feel like I don’t have it in me anymore. I’m a very average intelligence guy who had to work hard to get an undergrad and don’t think I would be able to do it again if I tried. It sucks honestly if this is it

I’m wondering if anyone here has actually had a breakthrough in a later age? I’m not talking an already intelligent guy who maintained or slowed the speed of decline. I’m saying some average guy who had a sudden increase in learning ability or something along those lines. Moving a level up cognitively at an older age basically and how it happened.

I want to do alot more but it’s just depressing seeing i’m cognitively limited and declining fast.