Do you mean teenagers? I took the SAT at around 12 and at 16, for instance, and scored considerably higher the second time, with no special test prep (just a couple of sample tests from a booklet from the library a few days before that second test). I think that did help a little, but already doing a third one would've been in the region of diminishing returns. The difference really was just the time, and high school classes.
I'd be surprised to see this pattern in someone a decade older.
