Brenda Drake's annual Pitch Madness contest is going on right now (you can check out the details here, but be aware that the submission window has closed!). The readers going through the entries and deciding what entries will move on to the agent round have been tweeting general tips on the hashtag #PitchMadness based on what they're seeing. There are some really good tips on query pitfalls to avoid, so it's worth taking a look.
I noticed that one in particular keeps coming up: people calling their manuscript young adult when they're really writing material that's more suited for a middle grade novel. It's more common than you might think. So what makes a YA novel YA and not MG?
1. Targeted Age of Reader
This one's a bit obvious, but let's start here. A middle grade novel is intended to be read by kids who are roughly ages 8-12. Young adult novels are targeted toward 12-18 year olds (although, as we all know, they are often read by people much older than that!).
As a general rule, middle grade main characters tend to be preteen or below, and young adult characters tend to be fourteen or above. (There's an odd gray area with thirteen-year-old characters that sometimes makes them a hard sell.)
That said, there are a few exceptions to this rule. For example, Rick Riordan's Percy Jackson books are all considered middle grade because the main character started out as twelve years old, even though he's sixteen when the series ends.
3. Word Count
You'll find many different opinions about what constitutes proper word count for each age range, but the general consensus seems to be about 30,000-60,000 words for MG and 50,000-80,000 for YA. Sci-fi and fantasy books in either can have a high range that's ten or fifteen thousand words above this.
4. Primary Focus
Readers of middle grade fiction are focused on their immediate environment--friends, school, family--and the books targeted to this age range tend to reflect this. A MG protagonist often discovers something about themselves by the end of the book.
In contrast, YA protagonists are figuring out things about the outside world and how they fit into it. They're influenced by the world outside their small sphere and often have to deal with adult problems.
Elements like swearing, drug use, graphic violence, and sex tend to be very light or nonexistent in middle grade novels, but they're all fair game in YA. This doesn't mean MG novels can't be dark or scary, but writers will do well to consider that most of the books for this age group are purchased by parents or teachers and they will often take exception to edgier content.
Knowing the correct age category for your manuscript will make it easier to target the right agents and editors, and it will also help you market your book correctly. For further reading on this subject, check out this post on the YA Highway blog or this one from Writer's Digest.