I think it's not all that rare, at least among children from highly educated families. FWIW, I was told by a parent of a child there that our local state primary school had enough children who were "reading Harry Potter" on starting school that they had a reading group of them. Perhaps 4 or 5 children out of an intake of 60 or so, aged 4.5-5.5? (DS was AFAIK the only one like that at his school, though, out of 30-odd; parents at his (private) school are on average significantly richer but perhaps less educated - I think the reason this anecdote has stuck in my head is because it's contrary to the usual assumptions here about state vs private school pupils :-)

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