Why not write a book which is as sophisticated as a book for an adult, but is about the concerns that teenagers actually have?
Older teens tend to write to me and say, 'Thank you for not writing down to teenagers.'
I eat broccoli. I think about the plot. I pace in circles for hours, counter-clockwise, listening to music. I try to think of one detail in the scene I'm about to write that I'm really excited about writing. Until I can come up with that one detail, I pace.
I write for teens partially to work out whatever it was that I needed to from my own teenage years.
Occasionally people ask me how it is I write different types of things, and my answer to that is it's very natural. You get bored writing one kind of thing all the time.