FIrst of all, it is not true for 18 year olds.
Second, the gap between projections and current at time of acquisition depends heavily on scouting budget, and on what projections you see.
All that said, internationals signed at the age of 19 gain fewer points before they stop gaining that do JuCo draftees signed at the age of 19. This gap is more pronounced between intls signed at the age of 20 and JuCos drafted at the age of 20, and intls signed at the age of 21 gain very few points at all.
By definition, of course, you see this in the projections, more validly if you spend more; otherwise scouting would be worthless. But the growth gap, no matter what you spend on scouting and therefore what projections you see, is HUGE.