They did not create a new market. Adults were killing productivity at their jobs at sites like Addicting Games and Candy Stand long before Smart Phones and Nintendo monetized the casual experience. The other half of their market is younger children. I would argue the DS and Wii have simply been the apotheosis of the Nintendo experience, because Nintendo products always brought in young children (how old were we when we were glued to our GameBoys and GBCs?), it's just now they've made such quality titles and enough varied titles to spark any child's interest, that they've really smashed the sales charts. The one area where they have broke new ground is young girls and that is an accomplishment. I never understood the power of the DS until I realized I had just played NSMB with my two little girl cousins who both had DS' and were both under the age of 10. I badly want to discredit Nintendo's accomplishment in this area, but I can't. The one thing I can think of is Tamagachi but even that is questionable.
Still, I disagree that gamers have gotten older or younger. The older crowd now just has an outlet for it on one of the big 3 console manufacturers. And me and almost all of my friends have been playing video games as long as we've been old enough to hold and use a controller. But the young girls crowd? Yeah, Nintendo created that one. But I think they're losing it to Apple.