You never get too old for Pokemon. Some people simply get too pretentious for Pokemon.

Quite the opposite, Pokemon gets better with age:

9 year old me: Derp! Get all the super strong attacks in there, cus things like Tail Whip are useless! Huh?! How the hell did Double Kick kill me when he was Tail Whipping all this time?!

22 year old me: Hmmm… He used Tail Whip to lower my defense…I can use my Growl to counter-act that by making his attack weaker and thus bring order back to the balance, now maybe I should use this X-Attack to give my Pokemon the boost it needs?

You can play it just fine as a kid, but as an adult, you can appreciate the subtleties much more. That's why Pokemon's so good, is because it works for all kinds of age groups. The moment people start saying it's for kids is the moment they lose their imagination and have no appreciation for entertainment.