I'm 18 and I have quit playing pokemon in 2007 after 4th gen came around and killed NB and adv. I wouldn't have quit based on age, I simply found 4th gen terrible, so I ended up moving onto other games. I would actually still play adv, although very few people play it now so I won't bother getting the new version of NB or anything.
The only reason I still lurk around on the forums (well, only in the non-pokemon subforums now) is because of the mostly mature and intelligent community, and I actually still keep in touch with quite a couple of people I've got to know back in the times of NB.
Also, I have never told anyone in real life that I had played pokemon or that I visit pokemon forums. People are very conservative in my area and think games are nothing but a waste of time, especially when you're older, and they'll keep insulting you in front of others for playing them; pokemon especially is considered very childish.
But hey, what can we do besides quitting, let's just keep trying to believe that whatever entertains us isn't just wasting time. On this note, although it isn't pokemon but it can be related to anything really, especially games, including pokemon, I recommend listening to that youtube vid of a 24 years old veteran StarCraft player's life and passion for games from the second link in my sig (it may be 2 hours long, but it's still amazing, just check the comments).