The average life expectancy four thousand years ago was twenty-five years. Before that it was even less. This means that the world we live in is the creation of young adults (many of them were children by modern standards).
Mankind’s achievements in the prehistoric age—from the invention of wheel, to the discovery of agriculture, domestication of cattle, development of languages, rise of the world’s first primitive religions, mythologies, philosophies, and political systems—was the outcome of the labor and enterprise of people who would be of ten to thirty-five years. A few people in the prehistoric age would live to the age of fifty or even ninety, but the lifecycle of most would plateau at thirty-five.
This implies that in the prehistoric period, which comprises 99% of human existence on this planet, the young adults were the driving force of civilization.
In the modern age, the situation has changed. In the last three hundred years, there has been a rise in average life expectancy and the forces of civilization have gone into the hands of older people. The average age in Japan is currently forty-eight years; in the USA it is thirty-eight years; in Germany it is forty-five years; in UK it is forty years. But when Japan, the USA, Germany, and the UK were in a high-growth phase, their average age was quite low. The rise of average age can be a cause of economic, militaristic, and political stagnation.
The young adults are often the best barbarians. The older generations are often the best utopians. History tells us that the barbarians are often the creators of new civilizations, whereas the utopians are often the destroyers of old civilizations.