An aging population is the inevitable consequence of longer life and lower birth rates. The median age of the world has risen from 22 in 1980 to over 30 today, and is projected to reach 42 by 2100.

The old-age dependency ratio compares people 65 and older to people of working age (15–64). A ratio of 30 means 30 retirees for every 100 workers. Japan is already above 50; most of Europe is above 30 and rising.

A high dependency ratio strains public finances: fewer workers pay taxes that fund more pensions and healthcare. Countries respond by raising retirement ages, encouraging immigration, or increasing productivity.

Some countries face the challenge of "getting old before getting rich." France took 115 years to go from 7% to 14% over-65; Brazil did it in 19, Vietnam in 18. These countries reach high dependency ratios at much lower income levels than Europe did.

Not all aging is bad news. Older societies tend to be more peaceful, more educated, and less prone to violent crime. The challenge is fiscal and intergenerational — designing systems that support the old without overburdening the young.