Now that's partly because I want more varied settings, partly because I was never fond of high school even when I attended one, partly because it's just more realistic for there to be more adults with that premise, but mostly because, in general, I just prefer watching/reading about adult characters these days. It's something I already realized when I took that Young Adult Literature class back in college. I'm tired of reading about teenagers. I'm past that point in my life, and now I'm more interested in people my own age.
That's probably a very obvious thing to say (and ironic considering that the story I'm writing right now has teenage protagonists), but it's true. That's what made Tiger & Bunny so refreshing to me. It was also refreshing when I rewatched Darker than BLACK recently. It was even true in Steins;Gate, though the characters there could hardly be called "mature."
I mean, obviously I still read/watch and enjoy series that have child and teenage protagonists. I'm not saying that I'm completely sick of those (I'd be missing out on a lot of great shows if I was). But these days, when I hear that yet another story takes place in high school, I'm more likely to go "ugh" and not give it a chance. :/