The median age of a population is the point at which half the population is older than that age and half is younger. In the last 50 years, the median age of the world's population has increased by just three years, from 23.6 in 1950 to 26.4 in 2000. However, median ages 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 African country of Niger has the lowest median age in the world at just 15.3 years (15.2 years for males and 15.4 for females). In fact, the vast majority of the countries with median ages of under 20 are in Africa. This is reflective of the fact that poverty, disease and ongoing conflict situations mean that life expectancy in many African countries is much lower than in developed countries. According to World Health Organisation statistics, Niger has an average life expectancy of 61.8. However, this is still quite a bit higher than that of a number of other African countries. Sierra Leone, for example, with its median age of 19 years, has an average life expectancy of just 50.1 years.
This is in contrast to highly developed countries such as Japan, where the median age is 46.9 years, and Germany at 46.8 years. However, it is the tiny, but incredibly wealthy, European principality of Monaco that tops the median age list at 52.4 years (51.1 years for males and 53.7 years for females). Not surprisingly, life expectancies in these countries are also high: Japan consistently has the longest-lived population in the world with a life expectancy of 83.7 (World Health Organization statistics don't include Monaco, although the CIA's World Factbook statistics put the overall life expectancy at birth for citizens of Monaco above Japan at 89.5).
It is also worth noting that median age statistics are also somewhat skewed by the fact that, at a global level, women consistently outlive men and therefore have higher median ages. This occurs regardless of country, although the differences aren't always significant. Furthermore, female-specific conditions (such as death in childbirth), can reduce life expectancy in women, particularly in developing countries.