I honestly hadn’t thought too hard about this question before and I still don’t have a good answer other than “I’m a young adult myself and so I just find it easiest to write for my own age group”. I guess not everything I’ve written would be considered YA–but I don’t know if it would be considered adult either. Most of my protagonists are somewhere in their teens. As I get older, perhaps my protagonists will get older as well.
Most of the stuff I read is YA, MG, or classics. I shelve books at my library and cringe every time I have to do the adult fiction section. It’s horrendous. So maybe I’ll try to fix that problem one day by writing adult fic. But at the moment, I don’t think I have the maturity to write for that audience. Nor do I really have any ideas for adult fiction stories. I guess that proves I’m not ready to write that yet–I don’t really have any adulthood experience to draw from.
"For He knows our frame; He remembers that we are dust." - Psalm 103:14