Changes in age structure are a consequence of the demographic transition.
Before the onset of the transition, birth and death rates were quite high and populations grew relatively slowly or not at all. The demographic transition began in most countries with a decline in death rates. Improvements in mortality were particularly concentrated among children and infants, and this led to substantial growth in the young population. In developing Asia, this transition occurred mainly in the 1950s and the 1960s. Then birth rates began to decline, and the large birth cohorts of the 1950s and 1960s began to reach maturity starting in the 1970s, fueling the growth of the working- age population: phase two of the transition began.