My brain power depends on my retained mastery of analyzing in detail what's happening in my world and in my mind and body. I must continue to practice to retain my constructive and analytic powers. The goal is to be a master of my environment.
If a brain is exercised properly, anyone can grow intelligence, at any age, and potentially by a lot. Or you can just let your brain idle - and watch it slowly, inexorably, go to seed like a sedentary body.