Anyway on to my post.
I've been wrestling with the idea of pitching my YA novel as Upper Middle Grade. While researching the topic I stumbled upon an interesting thread at YALITCHAT. In short, most people agreed that age was a factor, but not a deciding factor. Harry Potter and the Order of the Phoenix was considered MG (middle grade) even though it contained dark themes and Harry was fifteen. The romance that soon started to develop within the series was relatively tame as was most of the violence. Even though book 7 turned out to be YA and a bit violent it was in no way as violent as many other YA novels so it was still okay to advertise the Harry Potter books to middle grade readers.
Basically, a book can be considered MG as long as these older themes are tamed. Consider, Tamora Pierce's the Lioness quartet series. It was considered MG yet spanned the life of a girl trying to become a knight from age 11 to 18. Throughout this time she learned about love, sex, and how to fight and protect her people. Even though Pierce's novels could be somewhat violent it was always understated and any sex scene faded out. They were all tame, making the topics perfectly acceptable for MG.
YALitChat's Middle Grade lair also posted an interesting article from Upstart Crow Literary explaining the fine line between YA and MG. I found it helpful and think it may prove useful to others in the same boat.