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.
This makes perfect sense. I quit playing video games when I discovered girls. Didn't get back into them again until I was married. There is a big black hole in my gaming life from about 1992-2003. I pretty much missed out on the SNES/PS1 years because I was too busy chasing girls, driving my motor cycle and trying to make my band famous. Now that all that youthful nonsense is out of the way, I can get back to business. I've spent the last 5 years playing catchup :)