Young Adult Literature; Coming-Of-Age
This book is targeted toward people on the brink of their teenage years, and it deals with changing bodies, first kisses, and pressure to fit in. In other words, it's written specifically for young people, and as Judy Blume herself says, it is a book "I imagined a child would curl up with alone." So while it's definitely for the younger folks in the young adult audience, it falls firmly in the YA category.
It's not surprising then that this book is also in the coming of age genre. It follows Margaret on a journey of self-exploration—physically, socially, and religiously—and though she isn't an adult by the end, she has most certainly left childhood behind. She's changed immensely since the book began—plus she's finally gotten her period—so while she isn't officially a teenager just yet, our girl has definitely come into her own in some pretty major ways.