One thing to think about is that my kid was sick of the SAT by the time she graduated from high school. She had take the SAT or PSAT 6 times by then! She took it in 7th grade with no prep, finished in top 3 in our state. She was not happy with her math score, so prepped a bit (maybe 2 hours!) for 8th grade, and raised her score enough to finish 3rd in NUMATS for her CR score. Then... her school has kids take the PSAT in 10th & 11th grade. And she took the SAT twice to get the scores she wanted in her junior year. She was heartily tired of that test by the time she was done with the last sitting.
So... the long haul is something to think about. You could have a nine year old take it. But be sure you are thinking about why you would do it. Are you having trouble getting the placement you need for him? Do you need it for entrance to talent search classes or something like THINK? If it is just because you have had him testing every year, and this seems like a logical step, consider taking a couple of years off.
