It is slightly more subtle than that. Disease tends to kill children (and the elderly) preferentially. People who survive childhood are relatively resilient. In

It is slightly more subtle than that. Disease tends to kill children (and the elderly) preferentially. People who survive childhood are relatively resilient. In countries with poor healthcare, there may be high birthrates along with high child mortality rates with the net effect being low population growth rates and a fairly broad age distribution. However, once developing countries start to make gains in health, the child mortality rate starts to fall. Birthrates, on the other hand, start to fall with education (i.e. one generation later). The effect is a boom in the number of people surviving to adulthood and a large bulge in the youth population that is being seen in many developing countries.