The brain not functioning properly after the age of 20.

I’m about to turn 21, and the things I learn don’t affect me or stick in my mind like they used to. Of course, my phone addiction plays a role in this, but I’ve started to accept that I’m getting older.
Since I was 16, I’ve been experiencing hyperfocus, feeling like I could conquer the world, but now I feel more like an alcoholic dad.
What are your experiences with these ages?