ds was 3 and a half. I taught him 2 or 3 words and he started reading Mr Men books without any help. Three weeks later he could read in his head. I used to read to him when he was a baby, he had a bus for his first birthday from my friend which had letters and numbers on it (you press them and they make the sound), he knew them all before he was 14 months.
Dd1 started reading chapter books (Mr Majeika etc) by herself around the time she turned 5 (but that was freakishly early). Ds got to the same point around the time he turned 6, which still made him one of the best readers in the class. He came up to me with an air of one who has made a great discovery, saying, "Mummy, if I look at the pictures I get part of the story, but if I read the words, I get the whole story!" He was so pleased, bless him.