Well, adults often tend to simply DVR the show in order to watch it when it's convenient for them, which, among other things, allows to skip commercial. Second, you don't have that much time for advertisement, and it has to match the type of a show at least a little bit, so even if people watch them, sheer numbers still matter more.
As far as i understand, they make a lot of money selling supporting products, like DVD's, novels, figurines, etc. (remember Lucky Star
). This is something kids are more likely to buy, and they're usually persistent enough to leach enough money from the parents. Besides, some teenagers take part-time jobs entirely to support their hobbies, while adults plan their budget in a different way.
And actually, there's no guarantee that Spice and Wolf is popular amongst all the adults. It's quite unusual and I doubt everyone likes it...
