My DS has a January birthday, and was academically beyond K when he started at age 5 with agemates. He went to an all-day K, having had experience with 2 hours of preschool 3 days a week. He came home exhausted for a few months. Since 1st grade in our district is a "learn-to-read" and learn basic math year, we decided to do kindy with differentiation and then skip first. Our son also refused to learn to write before K, so it was a good place for him to learn something that everyone else was learning too.
Looking back (just a bit, as DS7 is a rising 3rd grader), I still feel that it was the right decision for us to send DS to K and skip 1st. For our kid, it was best to learn "how to do school" with other kids experiencing that for the first time too, rather than being dropped in with 1st graders who already knew what was going on. When he got to second, he was still underchallenged, but that's another story.
