Obviously, every kid is different, but if you just want to know what is within the realm of possibility, here is our experience:
DS5 started private kindergarten this year at age 4.5. He read easy readers when he was 2 and now reads at at least the 6th grade level. He doesn't have the stamina or interest to read big chapter books on his own, but he follows them perfectly when I read them to him, and he loves to read comic books, graphic novels and magazines on his own. He's also big into Greek and Norse mythology and likes history and science. In class, his teacher uses his language arts time to work on writing responses to questions, where he's probably at about a first grade level. In math, he can count by 2s, 5s, 10s and do addition and subtraction and some simple multiplication. In school, he's working on double digit addition and story problems involving the concepts of multiplication and division, which seems appropriate.
Had I allowed the public school to intimidate me (and they tried), DS wouldn't start kindergarten for another 7 months, and he'd be "learning" the alphabet and how to count once he got there. When I toured the public school, they proudly showed me a class of kindergartners using Starfall to learn the letter sounds. It seemed like a bad fit to me even back when DS was 3, and it is absolutely unthinkable to me now. If DS were to go to public school next year, he'd have to start in at least the 2nd grade.
