Middle Grade or Young Adult. Who is your character? Generally speaking, MG characters are between the ages of 8-12. YA characters are from age 12 and can be as old as the summer before college.
MG characters are inwardly focused, trying to figure out how they fit in their world. YA characters have realized the world is diverse, if not a little scary, and try to make sense of this world and their place in it. Of course, both types of characters grow in their new found experience and knowledge.
As there are no hard and fast rules, just write! A good story will make its own place with or without labels.