Of course, all individuals and their children vary, so you may well have a mature young reader who can handle it. In general, however, I'd actually bump this up to a 15+ read due to suggestive/coarse language, violence, and the overall development of the relationship between Patch and Nora, which could easily be construed as unhealthy. I wouldn't recommend an impressionable youth reading it unless you plan to discuss it afterward to explain (and possibly debate) what was appropriate and what was not in the way Patch treated Nora, Nora's response, and some of her overall decisions (a specific scene where she's in a bad neighborhood and makes the most asinine decisions concerning her personal safety come to mind).
I would say if this were a horror film it would get an R rating. It has stalking in it and the romantic love interest is basically a serial killer. So, keep that in mind and judge accordingly. I for one wouldn't give it to anyone under 15~16 since it's basically teaching pretty messed up relationships.
it's not appropriate for anyone of any age to be honest with you, unless you want to open up a dialogue with your sons and daughters that goes something like "NEVER EVER EVER TREAT ANYONE THE WAY PATCH TREATS NORA AND NEVER LET ANYONE TREAT YOU THAT WAY EITHER!"