My children all started to read when very young, they were aged about 20-22 months when they started.
All of them asked and I never made them sit down and do it, it was a game they asked to play just as anything else they played with.
There's a big advantage to them learning to read early. The early stage readers are actually interesting to a 2yo. To a 5yo they're dead boring. But to the 2yo the fact that Jane Loves Peter; Peter loves Jane, and they both love the dog (or whatever) does end up with lots of excitement from them.
I don't think it does set them up as brilliant readers above the rest of the class for all their school days, and it's not meant to. What it does is teach them at their level, when they want to, and you, as a parent can make it so much more individual-ds's first words he could read included "Concorde" and "Sonic boom". I would say they've probably settled down into roughly their natural space in the class round about year 2, from what I've seen.
I did teach them whole word recognition, but they all picked up phonics without any difficulties when that became relevant. No school has had an issue with them reading.
And my brother learnt to read (and write) before he could properly speak, because he had a speech problem. So he would write what he wanted to say, age 3yo.