I dislike that kind of thinking too... >>;
It's for ignorant people. Just because they see some images of pokemon which might look for kids (like pikachu and all those cute pokemon) doesn't mean that they really know what pokemon is.
I am 17 and although I haven't played the game in a while, I am still interested in Pokemon. It is fun, and it is not only for kids. For example, if you see certain features of the game regarding different battling strategies, you will understand what I mean by that... A kid would find that stuff too complex to be defined as 'fun'.
Ignorant people shall always exist, and if respect really existed, people wouldn't make such comments on other people's interests without really knowing it.
I too hate this kind of behaviour, and it does not only happen regarding pokemon, but in many other things I like (like me having a passion for Japan and people doing their ignorant 'chinese-like' imitation of the language or behaviour, or when people start imitating bruce lee's kiai and doing stupid moves whenever I talk about my passion, which is Ninjutsu [in fact, I don't ever mention it anymore to anyone as they seem too stupid to understand... >>;).
That's how life is, unfortunately.