Around their fourth birthday, I checked to see if my kids understood the concept of blending letter sounds to make words and they did. The public library had an audio version of the BOB books. DD loved that and listened to the first set over and over again in her room. We did maybe one other set of BOB books and then she was able to read easy reader books pretty easily. By the time she started K she was reading second-third grade level stuff and now in third she is a very fast, fluent reader.
DS never seemed as interested and I just kind of left him alone. I did do one or two sets of BOB books with him and occasionally tried to get him to read a Level 2 easy reader book. Despite hardly ever reading and having almost no interest, he somehow learned extremely fast and was reading second-third grade level stuff pretty fluently by the time K started. He is a master of phonics--can decode just about anything, even words that he's never seen or heard before. And he's very good at spelling. DD didn't pick up on the phonics/spelling quite as easily and seemed to do more memorization of words than DS did.
Each kid is so different and it's good to let them take the lead. I liked the BOB books, at least introducing the first set of them but there are plenty of similar easy phonics books you can get. I tried Starfall once of twice but the kids weren't really into it.
