In a recent conversation with Nintendo of Japan’s president Satoru Iwata, game designer Shigeru Miyamoto explained that Nintendo does not try to make what are called ‘kiddy’ games.
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.” he explained. Essentially, the way the games are portrayed and the audience is just the nature of the beast and they are certainly not intentionally built that way.
Miyamoto also noted that the speech the characters use comes into play with the audience as well, “When someone in their fifties like me hears their player-character speak childishly, 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.”
From what it sounds like, Miyamoto really tries to make games that anyone can play, regardless of age. I would say that is actually a rare trait these days as many seem to stick to their one audience and hardly vary outside of that.