Use economic determinism perspective for your explanation. A The teenager is a social invention, one that took shape during the first half of the twentieth century in response to a society very different from our own. While, for example, a bottle-making machine or a microprocessor is quickly supplanted when a better one is invented, a social invention like the teenager is difficult to displace. It has become embedded in the way we imagine the shape of people's lives. It has become part of the cycle that begins at birth and ends at death.
Nothing seems more real or more changeless than that.