There are several things going on here. First of all, achievement test scores, even from individual achievement tests like the WJ-III, don't indicate the level at which a child is truly working. When my son was 9, he scored well beyond the "adult" level in math calculation on the WJ-III. He had just finished 6th grade math and knew how to do simple "solve for x" type problems. This shows a whole lot more about the math ability of the average adult than it does about my son's ability. At the time, my son was working at a prealgebra level in math.
As for how far ahead gifted kids are in math, my older son (who has dyslexia) started out behind by about two years in 2nd grade. By the end of 2nd grade he was ahead a year. By the end of 3rd grade he was ahead two years and he continued to the two years ahead at the end of 4th grade (when he tested at an adult level). In 5th grade he moved into algebra I at half pace, making him (sort of) three years ahead. He finished algebra I at the end of 6th grade, so he was back to two years ahead. He did some "fun math" for about six months and then we messed around a bit trying to find a geometry program that worked for us, so he ended up finishing geometry in the middle of 8th grade, making him about 1.5 years ahead. Then again we messed around finding an algebra II book that worked for us. He is now on track to finish algebra II at the end of 9th grade, making him 1 year ahead.
My younger son, now 9, started out doing 1st grade math in K. He didn't seem particularly gifted in math and he moved ahead at a typical one level per year rate in K and 1st, so that at the end of 1st he had finished 2nd grade math (his WJ-III scores indicated that his math achievement was at a 5th grade level at that time). Then he started speeding up. He was two years ahead at the end of 2nd grade, and now in the middle of 3rd grade, he is doing a prealgebra program and is on track to start algebra in 4th grade, making him 4 years ahead. I anticipate that we will move more slowly through upper level math as I intend to heavily supplement the standard curriculum.
