Deciding the category?

I'm sorry, I'm new the forums but I'm really trying to find an answer to this question. Didn't see it in frequently asked questions. But I think when I finish my first novel I'm targetting towards the teen section in like barnes and noble. But as you can tell in the store things are divided into catogories at the store. Would you write your novel based on what audience you after or the genre? And if it's targeted towards teen would there be a certain age limit in the characters that you use? Any help would be appreciated.