Ogden Nash was reported to have said,
The trouble with a kitten is that
eventually it becomes a cat.
On this theme, the problem with Christian teenagers is that they become Christian adults. Thomas Berger gives an interesting perspective on this in this insightful article on the Juvenilization of American Christianity.
Juvenilization happened when no one was looking. In the first stage, Christian youth leaders created youth-friendly versions of the faith in a desperate attempt to save the world. Some hoped to reform their churches by influencing the next generation. Others expected any questionable innovations to stay comfortably quarantined in youth rallies and church basements. Both groups were less concerned about long-term consequences than about immediate appeals to youth.
In the second stage, a new American adulthood emerged that looked a lot like the old adolescence. Fewer and fewer people outgrew the adolescent Christian spiritualities they had learned in youth groups; instead, churches began to cater to them.