I don't know how old a child should be before they can totally understand this book but I think parents should ensure that all their children read/listen to it as soon as they are.
I listened to the audio book on Libby and it reminded me of my own issues when my parents moved towns forcing me to change schools at 14. As a "normal" child it was traumatic enough but I can only imagine how hard it would be for a child like Aven.
In a world where compassion seems to be at a premium it's always good to be reminded there are others that might not be as fortunate as ourselves. Like I said get your children to read it or if they balk at reading books let them listen to it. Not only will they learn something but they will probably enjoy it.