People like what they like. I know some friends that would prefere Mario and games like that over games like Gears of War 2 and other bloody, violent games. As for me, I basically just play stuff I like; and I don't like kiddie games. Mario and Donkey Kong were fun, but I eventually got bored with it (I still keep them around in case I want to play them again).
Peronally I believe everyone is entitled to their opinion and personal tastes. But if I go to a friend's house and they're playing Little Big Planet and ask me to play I would politely refuse. Not sure why but I cannot get into a game that feels "childish". I tried playing Fable and it felt so childish I just stopped. The game looks like one of those anime shows with over-animated characters with eyes as big as dinner plates. That's not exactly how it looks, but that is definately how it felt.
I agree with your reply that a whole "dismissing a whole univers as childish" is wrong. Some consider games childish, probably cause they think of games as Candyland or something. And some, like myself, consider it a new form of art and entertainment in the world. All the people in western society that gripe about what kids play is...misguided (IMO). Parents that blame games for stuff their kids learn. Well parents, there's a thing called a "rating" that comes with an "age limit" and (from the stores I go to) a kid who is eleven years old and looks eleven years old cannot buy Dead Space or Halo 3 without the parents there to give permission. While I'm on Dead Space, I had to show ID showing that I'm 17+ in order to get the game. So, as far as parents griping on games, look at the rating.