I'm in my 40s and feel sharp, with some evidence to back that up (such as career success and learning new things well). I believe these things help me stay sharp: I prioritize sleep. I try to get exercise (most often running) every day. I try to write every day; I write about ideas (typically new ideas in math or philosophy). I often read something I'm learning, such as a math book, something for work, or learning a new language. I meet weekly (online) with someone who's teaching me a new language - in this case French, but I think what's important is to keep learning. My work presents new technical challenges regularly, and I made a life decision to change my work whenever I feel burnout approaching.

To address other comments: I have two young kids, and I've been a single dad since my youngest was 3. Some commenters speak as if having kids kills your work or thinking life. This is partially true for the first year because of sleep depravation. And it's a new level of difficulty in terms of organizational demand on your life. It's harder. But it doesn't kill your mental abilities, and in fact I'm a happier person with my kids around, which gives me more motivation and life satisfaction. I've been able to solo run a successful bootstrapped startup while being a single dad. It's difficult and requires focus and deliberate life organization, but it's possible, and in fact quite rewarding and nice.

(Also, maybe the above makes it sound like I've had zero problems, but that's not true. It's been a long journey to figure out my priorities, with many mistakes and setbacks to being where I want to be. I have had times when I felt mentally less sharp, and worked to recover from those.)