"Unless you're careful, there's a tendency for games like Mario that can be played by children to gradually become childish," warns Mario creator Shigeru Miyamoto. "Those making the game tend to unconsciously make them that way."
According to to the famed game designer, the line "Where'd my mummy go?" was proposed for a Nintendo game. "When someone in their fifties like me hears their player-character speak childishly like that, it doesn't quite sit right," Miyamoto explains. "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."
Generally speaking, Nintendo and Miyamoto have historically been good at balancing kid and adult appeal. You could say it is the Pixar of the video game world — better yet, you could say Pixar is the Nintendo of the animation world.
Iwata Asks: Super Mario Galaxy 2 [Wii.com]