No one is too old, too mature, too "hard" to play Pokemon.
The people who typically tell me they think they're too old to be playing Pokemon are, oddly, younger than me. They are typically High School aged, and are completely foreclosed on brightly colored, light-hearted games like Pokemon. Instead, the constantly opt for "hardcore" games such as Bulletstorm, or Dead Space.
Not that there's anything wrong with those games, but variety is the spice of life, and there's no reason I can't enjoy explosive gunmetal-gray action in, say, CoD, and still enjoy the brightly colored joys of Pokemon. Yes, it's made to be kid friendly, and yes, as an RPG it's not the deepest, but there's no reason to consider yourself to old to play one game, and not another. If you enjoy it, you enjoy it. Forget the judgment of the uninformed.