The idea was to create a series of books that kids can grow up with. That's J. K. Rowling's intentions anyway. The point is ... They're supposed to read it as they age along themselves. By the time she gets to the next book, parents will strongly be urging them "NO! You can't read that until you're of proper age!"

But lemme tell you. I was reading things with content in it far more mature than what's in the Harry Potter books when I was 12 or so myself. And my parents didn't know any better because they neve read anything like that themselves.