If a novel is a Printz Honor Book, you can be sure it's young adult lit—after all, they don't give this award to dead white guys who write about all being quiet on the Western front. The narrator's a teenager, and except for her parents, teachers, and doctor, the characters are all either in high school or college.
A coming-of-age tale explores a young person's journey to adulthood, and even though Virginia's not an adult by the end of the book, she's dealing with adult stuff, specifically sex and romance. She learns that sex can be both fun (snogging Froggy) and dangerous (date rape), and she also discovers who she wants to be (an outspoken feminist) and what she's good at (writing). In other words, she's not quite there yet, but she's well on her way, so this book sits comfortably in the coming of age category.