Throughout human history, age patterns have followed a predicable trend: Parents would give birth to large numbers of children, not all of whom would reach adulthood; and as a generation moved from childhood to adolescence and through adulthood, more and more would succumb to death from illness, accidents, or conflict. The result was a pyramid-shaped age distribution, with large numbers of young people and fewer and fewer people attaining the age of 50, 60, or 70 and beyond.
