It was not that it can't work for all ages - but that it was its "target audience" for sales purposes. Whereas it was originally aimed at rather younger children, and is essentially a fairy tale. It may be more a reflection on the decline in reading as anything else. It may be inconceivable that a younger child would have the stamina for a book running to over 300 pages in the children's edition in these days when 16 year olds don't have to read the whole book even for GCSE (so I'm told).
At the risk of sounding like Methuselah's grannie there were so few alternatives to reading for my generation as a pre-teen that you just read more and perhaps were less likely to be fazed by a longer text. Or maybe I am just getting hopelessly middle-aged and so obliged to whitter on about such things
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Oh mercifully the whole vampire thing has passed me by... too old for Twilight..to tired to keep track of True blood...