of socially visible teenagers is a new one, and not, as the first writeup in this node assumes, a natural progression
of human development
Teenage culture exploded in America, and later spread to the rest of the world, in the fifties. The post war economic boom lead to teenagers being able to earn money for themselves while still being secure in the parental home, which turned them almost overnight into viable and rapacious consumers of all things radical. The advent of the pill lead to an increase of teenage sex coupled with a fall in the number of teenage marriages. Essentially, a period of life was created between childhood and adulthood which did not exist before.
Prior to that era, teenagers were either working for a living, studying, or married. Although a cliche, it is nevertheless true and important to remember that children were a part of the work force as soon as they could walk during the industrial revolution, and that children in the thirld world today still are. The average age of marriage was almost half of what it was today. People simply did not have the time to wallow in sullen misery for a few years before getting on the right track in life.
The obliteration of the teenage phenomenon, therefore, if indeed it is on the cards (and I can't see that myself) would be neither unnatural nor dangerous. It will simply be a return to normal patterns of growing up. Neither does the lack of strict discipline and open rebellion mean the shortening of childhood. Childhood, in the romantic sense we tend to think of it in, is much more in danger fom the media, the internet and rampant consumerism and capitalism.