I'm over 40 and even though I mostly manage/lead now I have time to do programming and plenty of math. I still see improvement mentally (not so much physically anymore), but also a lot of improvement in skills I neglected when I was younger like interpersonal skills and sales. I'm also learning a new language and read more than ever. Sometimes I feel like I'm less sharp, but I wonder if that's because I'm doing so much more.

My tricks that I don't always follow, is work out every day, get enough sleep, and stay off of most short form social media. I realized when I was on short form social it would zap a lot of time and kill any focus I had.