DS's summer birthday meant he would have been a young 5 or a young 6 going into K. (I can't have imagined putting him in as a young 4.) We opted for young 6 to give him an additional, wonderful year at his preschool, figuring we could always accelerate if it became necessary. We also had no idea he was HG.
When it came time to have the acceleration discussion with the school, it was met with a lot of resistance. They were basing his reading ability on a test he had taken several months previous (when he was reading slightly but not dramatically above grade level), saying that the last time they skipped a kid, he was reading at the 4th grade level. Of course, not a week later, DS's reading ability skyrockets and quite obviously goes well past easy readers. His WJ achievement scores that he took later in the year came out average 4th grade ability...so go figure. Yet another reason to trust the opinions of parents.
We ended up enrolling him in the first grade of a gifted private school for this year, so we stopped short of getting a grade skip at the public school. K was a great experience for him, though. It was half day which I think made it less dull for him, and he got to meet kids his own age who live really close by and gets along with well. So there was a lot of non-academic, valuable experience for him. It's too bad the academic experience fell short.
