Yeah I could distinctly feel my brain shifting into its adult era over the last couple of years (I'm 31)

It was kind of odd. I'm more serious now (but at the same time.. less?). I'm way more easily able to focus on what actually matters in this life. (In saying that, I think it's more likely that my brain has finally decided what's important... in a way I feel like a passenger)

I have the exact same feeling. Turning 30 was like flipping a switch for me. Since then, I've even felt like I've become more intelligent, especially in math. It's like everything suddenly clicked. I struggled with math throughout my entire school life, but now that I've gone back to college, I'm amazed by how easily I grasp concepts. I've recently taken on challenges that I always thought were exclusive to "smart people," like systems programming in Rust and Zig or building compilers.