As others have already stated, I feel my age itself (42) has zero to do with my decline in actual skills. If anything I feel I focus way better now than I ever have.
It is all of the other things that take time, such as memorizing patterns or maps, things were just outright "practicing" or playing brings an advantage, that has declined on my end. All simply due to not having as much free time.
Which is why I know I am no longer that competitive in the genres I tend to prefer (FPS's, action games, etc.). However I do believe if my current self played games like my old self used to, I would be able to hold my own decent enough.
With the above said, also as already stated, it has always been about having fun as much as anything else. But I would be lying if I said I did not enjoy being competitive back in the day. Especially with Counter Strike, Battlefield 1942 & 2, Joint Ops, and Unreal Tournament 4 specifically. I did try to be competitive in those, and I usually was able to do so.