Who reads young adult books?

An interesting article in the G&M today.
How Harry Potter rewrote the book on reading, in which we learn that in 2009 70% of readers of young adult books were over 18. Conjure up an image of a middle-age man reading Nancy Drew and the Case of the Twin Teddy Bears.

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Apparently, these 18 plussers show no shame in buying or reading these things.