The global demographic transition began around 1800 in Europe with declining mortality followed by declining fertility, trends which spread around the world and continue in this century. At the aggregate level, population size greatly increased, growth accelerated and declined with many countries now shrinking, and age distributions inevitably moved from young to old. Population aging has not yet run its course, Its effects exacerbated by declining retirement ages, straining pensions systems and prompting their reform. These aggregate demographic trends reflect profound changes in risks and behavior for individuals and families, and in the shape of the economic life cycle.
"The Demographic Transition: Three Centuries of Fundamental Change."
Journal of Economic Perspectives,