I think that a market and audience really shouldn't matter a whole lot until you're actually trying to sell.
And honestly, the lines are unbelievably blurry. I used to work in a bookstore and the whole labeling of age groups, genre, whatever - it doesn't matter. Commercial vs. literary? There are commercially successful titles across the board, so I don't think it matters much what it is you write, or what you want to call it. Genre or age group don't seem to relate to sales in the end.
Of course, labeling does matter - for obvious reasons; people want to know what they're getting into - but while you're writing it, not so much. Write it first, label it later.
While I, personally, may write things generally appropriate for the young adult market, they would probably fly in the adult market as well. (Dunno though, since I'm unpublished, haha.) Good to keep options open.
There's a huge difference between teen and adult books generally - but I think when you start getting into genres like fantasy, sci-fi, paranormal, things like that, the lines get really blurry. It seems that issues dealt with in those genres are quite universal, while more general fiction deals with issues specific to age. That's IMO.
