I guess it depends on how you want to teach it. If you are having the students do the majority of the reading on their own, and coming to class prepared for quizzes, discussion, etc. I'd say it fits nicely into British Literature (which is 12th grade in NC). However, I prefer to teach books really interactively. I do a lot of teacher-reading, paired reading, reciprocal reading etc, DURING CLASS. I've found (in public school especially) that students simply don't take the time at home to read. Even at the 12th grade level, I'd rather read to them or with them and guide them through the understanding than have them come to class having read nothing but sparknotes (or enotes!).
When teaching in this fashion, I think it is appropriate to do this text with as low as 9th grade even. I love the story, and have found that it is one that boys can get into, which is always a bonus for me. Dickens tends to be intimidating to a lot of students, so at whatever level you choose to teach it, I think it is important to provide context and relevancy clues throughout. You probably know as much as I do that even honors students simply don't put the time and effort into English class that is truly required for high level thinking. I always lean to the side of giving more time IN CLASS to accomplish the work, so I know they are actually doing it themselves. Perhaps that makes me an easy teacher, but my ultimate goal is to peak interest and get them to think, not have them regurgitate summaries and analyses from the internet, you know?