Most novels for that age group run around 40-60 K. Most first novels probably should come in closer to the 40K mark, just b/c publishers would rather take a chance on a new writer with a shorter book than a longer one - that darn bottom line.
Edit, then see where you are. As Torgo said, is it all exciting? If it is, you have two choices. One, submit it as is, and cross your fingers that someone will take a chance on a longer book by a new writer (which if the story is really good, you have an good chance) or two, split the book into two and try them as a pair. Books with sequels are in, especially with that age group. The trouble with it is that if the first book doesn't sell for some reason, you won't get the chance to publish the second.
Finish the book, really finish it, and then decide.