One of my professors at Uni was about fifty, and she put on this amazing steampunk outfit every month and attending a local monthly gathering, and it was totally cool. I don't think there's a limit, is what I'm trying to say.
Playing video games is scientifically proven to help prevent senility, so there's no reason to stop doing that when you get older, and conventions like that usually have a selection of, what I like to call, "veteran nerds".