There is some grey area when it comes to if your story is Middle Grade or Young Adult. In this post I'll walk you through how to decide what category your book or idea is, and what to expect in Middle Grade and Young Adult books.
At a Glance
MG is shorter (although there are exceptions). If there are issues in the book, they're experiencing them from a parent going through them, not usually themselves. Language is a little simpler with less complex sentence structure. Has a hidden message that explores themes and emotions. Ages 10-13. Word count is 30k-60k with the upper being due to a Fantasy.
YA is longer, edgier, darker, and is currently aimed at late teens. Ages 14-18+. Word count is 50k-100k+, but be careful not to have too high a word count, especially for your debut. The higher word count is usually for Fantasy. And 50k is on the very low end, 60k would be better.
The grey area is late MG to early YA in which ages and thought processes get blurred. The market for early YA (Teen) is lacking but hopefully there will be a presence again soon, because I think it's needed. There's also not a New Adult space, so sometimes adult content is seen in YA novels.
Middle Grade
Kids are thinking about the immediate and now. They're not too worried about the future.
They're concerned about their friends and family. They're trying to fit in and find acceptance. They're learning about physical changes, crushes, and school experiences.
The most romance you'll find is hand holding or crushes, maybe a first kiss. There is no foul language.
They're not as introspective or self-reflective. Characters react to what happens to them.
MG is faster paced, more action and less internalizing. This is a great place for big adventures. There is something that disrupts their life that needs to be fixed or restored.
MG is mostly written in 3rd person but don't let that stop you from writing in 1st.
Great themes to explore: friendship, family, belonging, loss, joy, sense of self, community, responsibility, hope. MG has a lot of heart and fun.
Young Adult
They try to figure out how they fit in the world and their relationship to it. They're figuring out who they are, individualizing themselves.
Introspective, emotional inner world. Focused on experiences and growth, with their choices, action, and concerns driving the story.
They reflect on what happens and analyze why. Think about the future. Feel pressures from their social world, family, friends, and themselves.
Upper YA has sex, violence, foul language, drugs, alcohol, etc. HOWEVER, please be aware that 14-year-olds will read this. If you think you're writing a New Adult or Adult story, switches ages, otherwise tone down all the 'experiences'.
YA is mostly written in 1st person or close 3rd person to really hone in on the internal thoughts and emotions.