Consider what your audience wants. It's too easy to write the book for you, the writer, but you have to consider what concerns your audience.
- Ages 8-12
- Care about personal loss (loss of parents, home, pets. They see bigger loss, such as 9/11, but are disconnected from the bigger picture beyond their person)
- Concerned about isolation and being alone
- Bullying is a big theme
- School issues is classic
- Gender issues arise, but it's more along the lines of "girls, ew!" not sexual
- Ages 12+
- Body image becomes an issue (Gratz mentioned that he didn't comb his hair until he was 14—before then, it didn't matter)
- Adulthood is a concern—questions about what comes next
- Sex is an issue. Before now, the boys felt funny about girls, but by high school, it's not funny any more.
- Drugs and alcohol do exist in this world. Your character or readers don't necessarily to do them, but they know it's there.
- Relationships tend to be absolute (OMG! I will never love again!)
- Religion becomes a question—no longer blind acceptance, but they begin to question their faith.
Now, these are general assumptions, and these issues do cross-over, but the important thing to think about is how the characters fit into these roles.
Gratz gave an example of divorce. A MG character would look at the divorce of his parents in an internal way: who's house will I live at, where will my stuff be, how will it affect me personally. A YA character would consider these issues, too, but he'd also question if there is such a thing as true love, wonder if dad is a bad person because he cheated, wonder if he loves one parent more than another.
As Gratz said, "Your story always needs to be about the kid. Find the child's story."