Some of the online stores that list Retainer Girl describe it as a kid's or teen book. Allie's eleven and I think she's still too young. I think it is definitely a teen book. And even if she were a few years older, I would want her to have started with braces before she read a story like this one.
I like to think that it's exactly the sort of book I would have needed about halfway into my orthodontic treatment, and that it would have helped me the rest of the way because it describes so much of what I was going through. But it's probably better to err on the side of caution. I think "mature teenagers" is sort of the minimum audience age. And the book is also meant for people who have been through it already, because I think older teens and adults can enjoy it too. And parents of kids who are going through it now.
I do like the title, but it really is a book about every part of having braces. Lots of looks back at the different stages she goes through. So I hope people don't just expect it to be just about the retainer period!