Actually, I think the bigger issue with the idea that the 9-year-old might be ready for MIT classes by virtue of having similar scores on SAT's and other tests is this:
The SAT's have a pretty low ceiling, and fail to capture a great deal of the knowledge that I would expect an MIT student to have. SAT math is not very high-level math. I would bet that most of those MIT students are adept at several years worth of math beyond SAT level and have a much deeper and better understanding of the topics that are on the SAT, plus many more.
Put me in the camp that is distinctly unimpressed by perfect SAT scores. Yes, if your 9-year-old gets a perfect score on the SAT he or she will need some pretty radical acceleration, but I'll bet there are quite a few of those MIT students who were exactly the same.
