There's no real age limit to playing Pokémon games, its when you feel the need to stop playing or that you've grown out of it. Like alot of video games, it's when you decide you don't feel like playing them no more, Pokémon is no different to any other video game, you see adults playing games like Halo, Call Of Duty and Pokemon is no different, young or old.
Pokémon has been a part of my life for, what.. 11 years now, so that obviously means something, like alot of people out there I don't see myself just stop playing anytime soon, it's a part of my life and will remain a part of my life, for lots years to come, especially the fact that I've played it for 11 years of my 17 year old lifetime.
