In my opinion, the book is most appropriate for Young Adults since the main characters are teenagers. Since it's a Christian one there's nothing offensive in it. But anyone dealing with autism will probably want to read it.
Unfortunately only a few few autistic kids are at the same place on the Autism Spectrum as Holden, the character who is helped greatly by music. But the primary focus is on the main character, Ella, who is one of the popular kids but decides to stand up for Holden in spite of what other kids think. I wish all teenagers would read it since they tend to make many bad choices because of valuing popularity.
The book is well written - I couldn't put it down - and I think it will make a difference in the world by helping "normal" people understand those who are different from themselves.