I think that about 30% of juniors reach the benchmarks relatively fast. Some take a bit more time, but usually are playing catch up then as the others had "it" sooner. Those who do not get it seem to either drop out or find something else to do.
I had thought in the past that in the USA the kids had an advantage due to Baseball and throwing sports, over those in other countries where they do not play baseball, football, basketball, etc.. Here in Germany soccer is the big game that the kids all love. None of my juniors have played the above sports, yet meet the benchmarks. My theory is that kids do not for the most part have the hand strength to mess up the benchmarks, and end up pulling the club through as fast as they can, which give them the lag which is necessary to become a great golfer.
The adults in Germany very RARELY meet the benchmarks, even the better players. I almost never see it. One thing that I have noticed is that those with stronger lead hand grips come closer to the benchmarks then those with weaker leading hand grips.