the pattern I see it this:
kids that go to college/university obviously don't pioneer right out of highschool. Typically, non-pioneer HS grads that go straight into the work force tend to marry good little JW girls young and fresh in their late teens and early 20's.
But if a college/univerisity student graduates at 22 years old, then spends a year or two focusing on finding employment, then their social network consists of mostly non-JW's and they are old enough to think for themselves and see the BOrg for what it is.
Good little pioneers would rarely have the mental capacity to think independantly of the BOrg and the 19 year old married couples tend to jump right into JW congregational life so they too, don't have the objectivity to question.
the reality is, if a young person delays "commitment" to the BOrg either by marriage or pioneering when they are fresh out of HS, their ability to think independantly of the BOrg walk them straight out the front door.... and this is exactly what is happening