I know a lot of people are against books like this being available for middle schoolers. But in today's world, with unlimited access to anything on the internet, the kids are already aware of everything in this book. Books like this are a good tool for them to understand sf form their own opinions.
I think if a reader will sit down and read this book cover to cover and enjoy it, then they are mature enough to understand it on some level. At preteen age, kids won't stick to reading something that makes them uncomfortable and the difficult topics brought up in the book are important to discuss. I know at that age I would be much more comfortable exploring mature themes in books than conversation. The framing and reflection Green writes with paints these mature themes as growing up and maturing into an adult. I say any kid in middle school or high school can read it if they feel comfortable doing so.