Too old is when you stop enjoying that moment when you start anew with the next Generation/game. I have been playing since 96, Blue being my first, and am 21 now. If you give up something you enjoy just because you are a certain age, then you are not living your life, you're living society's life. That's how I see it anyways, I still attend college, I still plan on getting my Masters, I still plan on teaching once I graduate, playing Pokemon, or enjoying anything at that matter, won't deviate my life style or choices.
Just my two cents. :P