Why write YA? How do you define Young Adult?

Obviously, age is a factor. You need characters who are, you know, young adults. For me that means anyone between the ages of 15-19ish, with 19 being on the cusp and story-specific. I also think there’s wiggle room based on say, inhuman characters. But a key has to be some sort of coming of age/realizing who you are, and choosing who you want to be. It’s something all teenagers struggle with, and it unites us in that time period. Characters need to reflect that. That has to be a major question of the story. Who am I? Who am I becoming? Do I like it? Can I help it? Can I stop it? And so forth.