I'll be 26 soon, gaming has been my main hobby since I was 4. I'd like to think that I'm as good at games now as I ever was, but there's no real comparison for me to make. I certainly feel I'm as responsive now as I was 5 years ago but humans also have selective memory. We remember things which aren't necessarily true (but not a conscious lie/mistake either).
The only real way to test it would be a game of skill and make annual records. For example, your average score of tetris across 5 games on a yearly basis.