Parents need to make this decision on behalf of their kids.
Although the title has the word “children” in it, this book was written for adults and mature teens. So much depends on the developmental level of the teens. I would say to use age 15 as a rough guideline, but I’ve meet a few kids in junior high who read it and really appreciated it. I’ve also had a couple parents take their teens out of my reading because they thought it was too heavy for them.
If you are not sure, I recommend reading it cover to cover on your own. That way, you have all the information to make your decision. One compromise would be to read it together and talk about it as you go along.
The exception is a child who shows initiative in persuing it. If your child has asked for the book, has asked about going to a reading, or pulls it off the shelf without any prompting, don’t stop her.