According to the latest UN reports, two-thirds of the global population live in countries with below-replacement fertility rates, while average lifespans continue to grow. This means that many populations are rapidly aging, and will soon begin to shrink (if they haven’t already). At the start of this century, 32 countries had a median age above 35 years. By the end of this decade, that number will more than double. And in 25 of those countries, half the population will be more than 45 years old.