Anecdotal: I seem to recall that I took it way back in the day (granted, pretty much a different test back then, in the days of the SAT-V, -M, and TSWE), in three consecutive years at about this age, and went up between 2-300 points over the span, with zero prep other than the little booklet that came with your registration packet in those days. That would be about 100 points in one year, mainly based on natural development and wide-ranging reading interests (and classroom instruction, for the math portion, as that was the period during which I had geometry and trig).
