This is probably not the best place to ask if early reading is rare. It seems less rare as we have met more families with HG+ kids. Although some very smart kids read much later.
DD now 7 read very early and it was hard when she started school. By 1st grade she had begun to choose books to try and fit in with the girls which were way below her level. After putting her in an HG program this year this has gotten a little better. I was very delighted after struggling to decide if we should let her read Hunger Games, because of content, that several other kids in her class were reading it as well! This was encouraging to us, and exciting for her. I worry about DS5 starting K in the fall. The school here seems to find early reading very unusual but we have met more and more kids who read from an early age. A lot of them don't go the traditional schooling route. Our school has an HG program at 2nd grade, pull outs in 1st and ability grouping and limited pullouts in K. It is still a big challenge to keep them engaged, and we did not meet other families with early readers until after DD7 was in the HG program. There were two kids who were reading in DDs K class which she started at 4.5, but they were not reading the kinds of things she was reading at home.
