This piece is very off-base, I think.
What is the point in learning the Gospel of Christ if they never get a chance to go out into the world and use it?
How many people in Church are hungry or thirsty or need helped out of the ditch?
I also think the 85% is a skewed number in that we are assuming that these kids are actually "the Church". Just because someone has been taught what the Gospel is, doesn't mean they were chosen before the foundation of the world, or if they were chosen, had been elected to believe at a young age. There is a difference between living in light of the Gospel as a Holy Spirit dwelt believer vs. living out a set of moralisms.
It is my understanding that the Doctrine of Perseverance will not have a true believer leaving the faith and never returning. If one does leave, it may be that they were never "of us"...but just "among us".