You're never too old for Pokemon. It's one of the best RPG and multiplayer games around. It just so happens that it was originally pitched at young children via the godawful anime (which really has very little to do with the real game besides the pokemon themselves), which imo misrepresents the Pokemon games horribly.
I used to, as people have mentioned, keep my private life and schooling life separate. Now I'm at Uni, noone really cares if I play pokemon. A couple of my mates even have the game, although they only played it for the story mode. As long as Nintendo keeps up the good work and delivers new, diverse pokemon and moves, and hopefully new game mechanics, I think people will play Pokemon for life.