Depends on the kid. Some kids are more grown up than others. I read full blown smut when I was 13. I only got the borning and akward talk from my parents. So I had to educate myself.

I honestly do not think it harmed me and think it is better to educate children sooner on sex than later. Books can be a great way of doing that. And I also mean by that books with problematic stuff like rape, unconsensual stuff. I think if parents talk with their children about such stuff portrayed in media it can help them handles this stuff better in real life.