I mention this because there are so many other factors in today's educational world. Homework is a big thing- alot of schools have lots of homework. Some of the homework is boring- you have to just hunker down and do it. I can imagine that a young-for-their-age child may have trouble doing it, not b/c they aren't smart enough but b/c they don't yet have the patience to plow through something tedious.
There is also the speed part- kids are expected to read/do math quickly. Again, it's not enough to just know the answer; you have to do it fast. Etc.
I'm sure some of it depends on what your school can offer your child. If they can't offer much, you might as well skip them a grade or more. Our local school has a great G/T program starting in 4th grade. By 8th grade, they will have skipped 2 grades in math compared to the regular curriculum. Our local h.s. offers 24 AP classes. If you skipped a grade early on and thus missed getting into the G/T program, I think that wouldn't be a good idea. On the other hand, if your G/T program is a joke, maybe it doesn't matter.
