Here's an interesting article I found online that I thought might be helpful. Nothing in this post was written by me.
Harry Potter in all its iterations means adventure, romance and mystery in a magical setting. On the other hand, The Earth, My Butt, and Other Big Round Things is funny, coming-of-age chick lit, intertwined with a brother’s dark secret. Both are Young Adult books, but YA is as broad as a category can get. Here are some general guidelines.
Young Adult readers are generally 12-18 years. The younger portion of this age group is often reading books that their parents would remember as teenage novels. But by the middle of the adolescent years, and sometimes earlier, most teens are reading adult novels. They get pulled back to YA novels with stories that relate directly to their own deep concerns, books that help them figure out their place in the world in a sensitive way.
Young Adult Genres and Subjects
As a group, YA readers will devour any genre: adventure, romance, humor, mystery, historical, contemporary, fantasy, sci-fi, etc. Various topics let teens explore unfamiliar aspects of their world and other cultures, bring bits of history to life, and experience things that they normally couldn’t. The underlying themes, regardless of genre or topic, allow teens to examine deeper issues in a safe way: what their role in life is, the difference one person can make, the importance of relationships, coping with tragedy of any sort, etc. The younger set of YA readers can cope with scary subjects when they are at a distance—the character’s friend is doing drugs, not the character himself.
When formerly taboo subjects are handled with an intense perspective, it is considered Edgy YA. There are no forbidden subjects here, but they are written with sensitivity and care, not gratuitously. These books are aimed at older teens. Instead of a friend or acquaintance having issues, the main character is the one being abused, cutting, considering suicide, etc., or it’s a family member or best friend of the main character. The viewpoint is very close, the bond and introspection and questioning are strong. Overall, teens can identify keenly with the character’s feelings, if not the situation.
The exact age of a YA character isn’t as important as the need for the age to fit the character. High school freshmen will act, think, and relate much differently than seniors. The most important requirement for your characters is that they are real. Their language, relationships, worries, hobbies, etc., must be exactly right for the person you have created. The second most important thing is that your story is written about the character and his or her experience and growth, not about the theme you have chosen. If you write the story well, the theme will come through.
Length and Style
YA novels generally run 40,000-75,000 words, but you’ll find books on either side of that. Write the story in the length it takes to tell it, and then check publishers’ guidelines.
Writing for Children
Each age category of children's writing, from picture books to young adult, has its own requirements. Follow these guidelines to start writing your children's book. There are no vocabulary restrictions for this age group. Watch out for stereotypes in your descriptive and narrative words. The acceptability of profanities and obscenities is determined by the age level and topic, whether it is necessary for the character to use them, and by the editor’s/publisher’s preference.
YA stories are usually written in first or third person (I said or he said), but there are some successful books written in second person present (you say). Again, it will depend on your style and the voice of your character.
First, read a large number and wide variety of YA novels, and analyze them. Second, write the best book you can. Third, study publishers’ guidelines and book catalogs, and submit to houses whose needs match your particular story.
Source: Writing Fiction Suite 101