Hard to say. I have not personally known any three-year-olds who read, but a number of five-year-olds who read (myself included). I'm trying now to remember when The Progeny started reading. She's no Trevelyan (which is a good thing for her); I'm thinking five. I don't think I personally knew any four-year-olds, either.

What I really don't know is how early kids can move from the decoding activity that is reading to a code that essentially requires two decoding steps: 1) put in regular letters and 2) read those. I wonder: if you teach a child codes/ciphers young enough, does the child simply combine the two steps in his or her head? I don't know. I didn't try codes until I was much older, and I'm not very good at them.