"Unless you're careful, there's a tendency for games like Mario that can be played by children to gradually become childish. Those making the game tend to unconsciously make them that way. Taking how Mario's line can be scripted as an example, they even propose to write, 'Where'd my mommy go?' When someone in their fifties like me hears their player-character speak childishly like that, it doesn't quite sit right. After all, Mario wasn't a game only for children in the first place. As I make a game, I try to keep in mind that guys in their fifties will play it, too." - Shigeru Miyamoto
Mario, childish? I don't think so, though there is some sort of perception that all Nintendo games with colorful graphics and cute characters are for children. You can blame the 'mature' people online in Halo 3 and Modern Warfare 2(note: 'mature' people who play those games that are under the age of 15). Hey, Nintendo has always been family-friendly.
As for 'where'd my mommy go?' it'd sound deeply wrong for Mario to say it, but if it was a Luma, that's just perfectly understandable. It's not childish.
