Our main character, Meggie, is twelve years old, and we're totally rooting for her as she learns new things about the world and herself in order to overcome the forces threatening her family—yep, we're pretty firmly in young adult lit territory here. Meggie comes into her own as a reader, and as a young person struggling to learn the truth about her family, like why her mom actually disappeared all those years ago.
Why fantasy? Some of the critters in the book are fairies, people made of glass, and monsters made of ashes… in other words, not creatures from our (real) world. The presence of magic is also a pretty big plot element, since Mo's (and later, Meggie's) ability to read people into and out of books is what makes the whole story happen. And this means that without magic there'd be pretty much zero plot in this book. Period.