Both of my daughters were reading at four. Both my sons started reading at six.
An interesting side note: My older son has Asperger's Syndrome and I suspect he may have been reading earlier and I didn't know it. As it appeared, he went from not reading at all to reading at high school level practically overnight. He was reading adult material - newspapers, magazines - at the age of six. And I don't mean slowly sounding out words. He was reading them quickly and with comprehension. It was really bizarre.