There may be reasons why you'd want to take the SAT math when very young (e.g. early admission to SET, or some kind of advocacy leverage), but if you're interested in tests for a gifted young math student, I'd think that the MAA tests (AMC8/10/12, AIME, USAMO) would be much more appealing than the SAT/PSAT/ACT. While the SAT math tests material that is "advanced" in the sense that it covers material that isn't normally taught to younger students, the questions themselves are not challenging to a good student who has seen the material. The MAA tests go to a deeper level of understanding and give a much better taste of what math is really about.
[link]
http://amc.maa.org/[/link]
