I was an early reader, as well, just like my two sons.
I think I was about three, although my mom didn't make a huge fuss like moms do nowadays. Back then you just got on with life; nowadays parents have to analyze and compare everything, and get it all on video.
I do remember reading in school. I couldn't understand why, when kids read aloud, they said the words in a slow, choppy way, or sometimes said the wrong word completely. My AS son was the same way; when his class was reading "The...cat....sat....on....the....mmmmat" he was reading long, complex sentences with inflection. The teacher taught the class to touch each word as they read it, so for a while he regressed to choppy reading. Then he refused to read out loud at all.
I think there is some kind of reading gene. It came so naturally to myself and my kids (and my mother), that there was never a time when any of us had to think about it. There was no conscious process of learning to read, and school had nothing to do with it. We just read. Then there are other people who really struggle. I don't think it's an "intelligence" thing, per se, either, because there are plenty of people who are obviously
intelligent, who need to go through that process of sounding out words.
Now...try to teach me to dance, and it's a whole other thing. Show me the world's simplest dance step a thousand times over, and I won't be able to even begin to copy it. I'm totally, utterly lost. So, I've got the reading gene, but not the dancing gene.