The median age of a population is the age at which half the population is older and half is younger (or, alternately, the age at which a person is roughly halfway between birth and death). For example, the CIA World Factbook estimates that the global median age in 2020 was 31.0 years, meaning 50% of the global population was younger than 31 and 50% were older. However, median ages per country vary significantly across the globe, and are influenced by a number of factors, such as birth rates, social and economic development and average life expectancies within individual countries.
The global median age of 31.0 in 2020 is a notable increase from its value 70 years ago, in 1950, when the median age was 23.6 years. This is a general sign that advances in medicine and infrastructure are enabling people to live longer as a whole. As a rule, median age is highest in high-income countries with a high degree of human development and overall standard of living. Similarly, it is lowest in underdeveloped and least-developed countries with lower incomes.
An example of this trend is Monaco, one of the tiniest, but also wealthiest countries in Europe. Monaco may have no military, but it boasts the highest median age in the world at 55.4 years (53.7 years for males and 57.0 years for females). While Monaco's data could arguably be skewed by its high number of wealthy retirees, the same accusation could not be leveled at the countries that follow it: Japan (median age 48.6), French territory St. Pierre and Miquelon (48.5), Germany (47.8), and Italy (46.5).
An even better example of the correlation between median age and income/development appears at the other end of the spectrum, where the continent Africa is home to most of the world's poorest countries and also to the vast majority of countries with median ages under 20. The African country of Niger has the lowest median age in the world at just 14.8 years (14.5 years for males and 15.1 for females), but many of its neighbors, including Uganda, Angola, Chad, and Mali, are close behind.
A country's median age is intertwined with its life expectancy, which also tends to be higher in wealthy and developed countries and lower in poorer and developing countries. For example, the Japanese consistently rank as the longest-lived population in the world, with a life expectancy of 83.7 years. Conversely, Nigerians have an average life expectancy of 61.8 years. However, this is still notably higher than that of a number of other African countries. Sierra Leone, for example, with its median age of 19.1 years, has an average life expectancy of just 50.1 years.
At a global level, women consistently outlive men, leading to increased life expectancies and higher median ages. This occurs regardless of country, although the differences are more significant in some countries than in others. Furthermore, female-specific conditions such as death in childbirth or the historical preference for male children in China can reduce life expectancy in women, particularly in developing countries.
Monaco has the world's highest median age of 55.4 years.
The youngest country per its low median age is Niger. That country has a median age of 14.8 years.