I see no reason not to take the SAT regularly from age 13 or so, if you can afford it. Getting comfortable and familiar with it before it "counts" can only help. I think the common counterargument is that it means that all your "younger" scores get sent to colleges, but (1) I think you can choose which scores to send and (2) I think that the colleges are smart enough to notice that you took it as a middle schooler or high school freshman that realize that your score would be expected to go up as you get older.
