Our child was 6 when he entered half-day Kindergarten and was way beyond it academically. We didn't actually realize he was so far ahead until recently, though (WISC-IV GAI > 99.9 percentile and WJ-III Achievement results consistently 2-3 grades ahead).

Believe it or not, he still enjoyed himself and we considered the year to be a success. It was valuable for him to learn the routine of school, he enjoyed some subjects immensely (like music and gym), and he made lots of friends.