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Rowling stated in an interview with Movie Magic:
"What I find interesting," she continues, "is that people call me a 'children's writer.' When I started writing, I had never thought of writing for children. I'd been writing almost all my life. I mean, the first story I ever finished, I was six years old. All I've ever wanted to be is a writer. I'd never thought of myself as a children's writer."
The Harry Potter books are marketed for children because they are about a child, but as the series goes on, there's more and more stuff you wouldn't want your seven year olds to read about...