Are we counting words or books? I definitely was able to read a children's book before first grade at age six (something to do with a fall birthday). I know this because I entered first grade knowing how to read and being annoyed with phonetics. I could recognize certain words at least by age 5 or 4. I can't get more precise than that because my memory kind of fails me, and evidently everyone else's does too.
I know I couldn't read at age 3 and it's pretty impressive that anyone could. I have memories from about that age being mystified by writing.