By keeping them young and not letting them explore their limits of what they find suitable or not, too adult, too scary, upsetting, uncomfortable, sexual or confusing we are setting them up for a big fall. I want my kids to know before they see objectionable stuff if it is going to be upsetting for them and that comes from testing the waters a little. Not every kid is going to want to read age inappropriate material and that's great. But I think with a little guidance the child doesn't have to face being blindsided with traumatic material.
I don't know what today's equivalent is but where would some us be without Judy Blume? I remember reading Forever at 11 and I think it was pretty instrumental in helping me make good choices when it came to boys.
I really agree with this. Good points, especially regarding the Internet.
Re Harry Potter, I think due to the popularity of the books, we all know the content and that later books are dark and contain scary and sad parts. But many, many children's books that are less widely known also do. (DD read some especially grim historical fiction in school in 2nd and 3rd grade--and that was based on reality.) Some of them are aimed much younger than HP. I find some of the worry about HP a little misplaced and suspect it comes about because as adults, we're likely to have actually read those books.