DS started reading early reader books (three sentences per page situation) around 3.5/4, reading slightly more per page around 4.5, strong reader for a Kindergartener, was reading Harry Potter first book in 1st grade. After that it's a blur because he's a voracious reader (is now in 7th grade).
I read to him from about birth and when I read, I ran my finger under each word - a lot of it was super simple text (letters, numbers, colors, spacial, short text books). Could've had nothing to do with him being an early reader. Its a crapshoot.