Hmm. I stopped showing off my pokemon to friends back in 6th grade. Of course now that I'm older, quite a few people in my age group (including myself) meet up to talk about it. I think there's a period in our lives (like, starting around 12 or 13) when it just don't seem "cool" anymore (even though we play it in secret), and then once we hit an older age where we just don't care as much, we go back to playing it without shame.
I personally don't think you can be "too old" for Pokemon. While the demographic is largely young kids (especially for the anime), I can see adults enjoying it as well. I mean hey, back when I got Yellow as a birthday present my mother got hooked on it.