We have a Sept. 1st cut-off and DD missed it by a couple months. We put her in K when she was supposed to go, so she was one of the oldest in the class (although one boy was still 6 months older! Held back). We saw rather quickly that it was not going to work out academically for her so started the process for acceleration. At that time, the district was actually open to it. Now they are much more rigid (just as an aside). So in late Nov. of that year, she went to first grade and was the youngest in that grade. The next oldest kids were a few months older, but most are more like 6-15 months older. She was high average for the rest of that year, but then started surpassing the older kids, and is now one of the top two out of about 80 kids in terms of achievement testing. I think age is a factor for younger kids, but after second or third grade it doesn't matter much anymore. What really mattered for my DD was the fact that she missed an entire year (most of kindergarten, and a few months of first grade), but even that didn't matter after about 6 months.
That said, most standardized tests for cognitive ability will give both an age percentile and grade percentile. If it's a straight IQ test it will probably only give an age percentile, it won't be based on grade at all.
