I don't think you can ever be too old to enjoy the concept of Pokemon, provided that you turn a blind eye to its many flaws.
However, I think a mature and intelligent brain can become bored very quickly from playing such a simple, repetitive game unless you spend the whole time calculating the damage. If ever I play a Pokemon game it's for the nostalgia effect, and I think if you get to roughly 10 years old without having played Pokemon, it's too late for you. The concept of Pokemon is brilliant, however, and it would be nice if the target audience eventually broadens to accommodate brainier geeks who require a significantly deeper exploration of the Pokemon world to satisfy their minds.
It would be wonderful to explore an open world like in assassin's creed and grand theft auto, it would bring some proper fantasy back to gaming, and it would be even better if they could make it non-violent. The worst thing about games at the moment is that, aside from sports games, it's all about killing people. It's unpleasant and so boring.