A creative writing major could focus her thesis on the differences between young adult and middle grade novels. There are tons of articles online covering this topic in depth. My goal here is just to provide you with a hard and fast reference to glance at when you're trying to decide if that SNI tiptoeing around the corridors of your brain is one or the other.
I hear you. "Duh." But it's not the age of your characters, is the age of your readers. Granted, usually a book starring a preteen main character is aimed at preteens and (even more so) younger. That's not necessarily the deciding factor, though – Janice Hardy's character Nya is fifteen in THE SHIFTER, which is middle grade, while Kirsten Hubbard's character Grace is fourteen in LIKE MANDARIN, a definite young adult. When you think age, think age of the reader, not character.
There's no romance in middle grade. I mean, you didn't have crushes at that age, right?
Both middle grade and young adults can (and often do) have romantic storylines. The romance in a middle grade tends to be sweeter, more innocent, like you were in seventh grade. (No, please don't tell me what you did with Jason Hunter under the slide.)
Young adult romances can be (but do not have to be) more sexual, both in feelings and actual action. Middle grade romances are more about the butterflies, the hand-holding, the first kiss.
This one is less obvious. Middle grade characters are focused internally; it's about self-growth, learning who you are. Young adult characters are focused more externally, noticing the world around them and how they fit in, how they affect things. Often, that's a huge part of a YA character's growth throughout his or her story; moving from a naturally selfish stage in life to becoming more aware of the feelings and situations of others.
Fantasy, dystopian, contemp, literary, even horror – there are middle grade and young adult books in all these genres and more. But both tend to have more favored genres. I'll let Stephanie Lane Elliott, Senior Editor with Delacorte Press, explain:
"Storywise, too, I think you see a difference between YA, where the characters are old enough to be pretty independent and get into trouble on their own, and middle grade, where kids’ lives are still fairly controlled by their parents—and so you see a lot of fantasy and magical realism. In middle grade, I think a lot of the action tends to come from imagination, whereas in YA, it’s tends to be a little more gritty and realistic."
And here we are – the dreaded "edgy." For lack of a better word, YA stories can, but do not have to, cover "grittier" topics. Rape, sex, drugs, drinking, abuse, etc. MG, not so much.
I'll wager we can all come up with exceptions to all five of these points. And that's what they are - exceptions! Generally speaking, if you aren't so sure whether your current project is YA or MG, I think asking yourself - what age am I writing for? what's the romance like? where's the focus? what's the genre? is it...naughty? - will help you figure it out.