I can't actually remember a time when I couldn't read to some degree or other, and my father says I sort of worked it out for myself around 3 or 4 years old.
My mum's favourite cutesy story about me learning to read, is that I was three when my little brother came along, naturally causing Mum to be dead tired when reading me my bedtime story. So she started missing bits out to get through Mr Men or A.W. Awdry quicker -- and I noticed and started watching the book while she read to make sure she didn't miss words out. (I must have been a nightmare kid!) Eventually I could tell which word she'd missed out just by the look of it, and the rest was history.
Peter