IMO
The Wii have a very WIDE range of audience, from kids to old people, and very diverse at that age too. The thing is, the Wii have more casual gamers than the HD consoles. Look at how many WiiFit and Wii Sport, and those carnival games that have been sold, we can estimate that a large amount of casual players bought those. And by casual, I mean those that have not game before and just got into the Wii craze.
The audience for Wii is a bit more different than those of the PS3 of 360. It takes a different approach to become successful of the Wii. Light, colorful, and cartoony games with simplistic but deep gameplay is the appeal of the Wii, anything else is a gamble if not done right.