I wrote YA partly by accident. I started a book with the same protagonist but 10 years older, got stuck, and decided to write a "short story" about his background as a teenager joining the circus. That became Pantomime. I initially subbed it as adult as I wasn't 100% sure it was YA. But that's what it sold as :-)
I've always planned to write both YA and adult fiction. I'm working on an adult SF thriller right now.
I haven't really come across that much YA prejudice, however I think it might make it harder to get mainstream press coverage on my books.
