Go on. Guess. The elderly? WRONG. Teenage girls? WRONG. Japan's biggest users of the DS are (according to this Nintendo-supplied chart, anyways) the same who were the biggest users of the GBA, and the Game Boy before that: boys. Specifically, 10-12 year-old boys.
After that, though, it gets interesting. The most dominant demographics after the boys are women in their early 30s, followed by 10-12 year-old girls, followed by men in their early 30s. Not teens, not twenty-somethings, but people in their early 30s. Anyone in charge of putting the questions together for their Friday night pub trivia competition, these ones are on the house.