For MG kids they are solidifying their own identity, experiencing the physical and psychological changes of puberty, taking on new responsibilities all within the boundaries of their family, friends and neighborhood.
Yes, your character needs to grow and change during the course of the book, but these changes are on the inside.
Middle grade readers are beginning to learn who they are, what they think. Their books need to mirror their personal experience.
The age of the main character and length of the manuscript are still a rough guide in determining the audience (middle grade manuscripts tend to be 100 pages or shorter, with young adult books being longer, though this is not always the case), but the kind of conflict the characters encounter is a better measuring stick.
The story, rather than the character's age, delineates the audience, as in Carolyn Coman's What Jamie Saw (a 1996 Newbery Honor Book).
The book features a nine-year-old protagonist, but the subject of domestic abuse prompted the publisher to give it an age range of ten and up.
As an author, it's your job to decide who you want to reach with your book -- elementary kids, junior high or high school -- and then create characters and conflicts accordingly.
Regardless of genre -- science fiction, mystery, historical or contemporary -- if your characters are learning about themselves and the world in the same way as your readers, your audience will find you.