DS could read independently before he started preschool, aged 3. He could read well before that actually and knew all his letters by 16 months. The best thing is to have the alphabet on the bedroom wall. I never pushed him but it's there in bright red and he always wanted to go through the letters whenever we were in his room. We also have letter blocks so we'd match them up.
From there he really just needed to be told about a few of the combinations, "sh", "th", etc, and he was putting words together in no time. We wrote them up on a blackboard or just paper and it stuck so quickly.
He had access to a word document on the laptop and enjoyed typing sentences out on there with a lot of help, me asking him to guess the next letter of the word he chose and so on.
It really was so easy and took no more than a few minutes each day.
All that said, he does have a bit of trouble settling at kindergarten and they think it's because he's "operating on a higher level" and thinks the other kids games are a bit daft. Do I think it's because he can read? No. I think he can read because he's a clever kid - I don't think he's clever because he can read. See the difference? Before he could read, when we went up the park he preferred having conversations, exploring, collecting, playing proper games and so on, while other kids mindlessly went down the slide over and over and over. He's just very bright.
Now my two 14 month olds are showing an interest in learning to read, picking up letter sounds. I have the choice to deny them the joy of reading at a young age in the hopes they socialise well, but I seriously doubt it would have that result, regardless of what mumsnet thinks. I'm pretty sure there's no rule that educated kids can't play with other kids.