The book includes a large quantity of information on Norse mythology, similarly to Rick Riordan's novels, except that Riordan's are Egyptian and Greek. Dissimilarly to Riordan, the books also take place during the Viking Era, rather than the modern era. The book is well written, and uses a somewhat advanced vocabulary. The estimate of ages 7+ was mainly if the reader is rather advanced, or is having the book read to them. Overall, this book, if you plan to go in-depth with the text, is best for a middle school class. They will be capable to understand the issues addressed in the novel, and yet not at a point where the actual book is boring.
One warning, there is some violence in the novel (In the middle of the book, there is a scene where the Vikings go berserk and murder everyone in a village. The actual violence is brushed over mostly, but the aftermath is described.) If you are particularly worried about your child hearing about violence, or you don't care to hear it yourself, then I would suggest skipping that section. A good trick for that if you are reading this to your child is to read a few pages ahead so you know when the section is coming.
On a humorous note, why would you read about a sea of trolls when you can simply visit the internet and experience one?