I am nearing that age, and personally I find its a factor of motivation more than anything. I got really excited about a side project last year and smashed out more code than I had in years. Refactoring an api for work doesn't sound super exciting. When I don't care, my brain doesn't care as much either.

If you do not believe that is a factor then examine your sleep and physical fitness. These things factor a lot more into brain function as we age.