Youth Fiction

I recently finished reading The Book Thief by Markus Zusak. I absolutely loved it! I don't want to say too much about it because I hope that those of you who enjoy reading will read this one. The story is great, but what I really appreciated was Zusak's style. It is unique. I enjoyed it so much that I have to ask a question. Why is it considered youth fiction? I am an adult and an undergrad English major and I found it to be very engaging. Is it called youth fiction because the protagonist is a child? When I think of youth fiction I think of children's books. What does youth fiction actually mean?