Let me chip in here as someone who has published three YA novels. I didn't necessarily write them as YA (although I had a publishing contract from a YA publisher after three chapters), I just wrote about teenage characters (18-20 years old) and wrote in my normal style.
This may have been a mistake. One reviewer said of my third book, the only audience for this book may be "literate, sophisticated and cynical 16-20 year olds." I actually liked that review, but I'm not sure there are enough readers in that demographic to support a novel, let alone a career.
To write a YA novel you have to simplify your sentences and your language. Subject matter is pretty wide open. But reading level is something you have to think about. If you're writing about a teenage girl in her tone of voice, with her language, you're probably writing something that could be YA or adult, depending on the complexity of the characterizations and the story.