Demographic window is defined to be that period of time in a nation's demographic evolution when the proportion of population of working age group is particularly prominent. This occurs when the demographic architecture of a population becomes younger and the percentage of people able to work reaches its height. Typically, the demographic window of opportunity lasts for 30–40 years depending upon the country. Because of the mechanical link between fertility levels and age structures, the timing and duration of this period is closely associated to those of fertility decline: when birth rates fall, the age pyramid first shrinks with gradually lower proportions of young population (under 15s) and the dependency ratio decreases as is happening (or happened) in various parts of East Asia over s