Demographic divided is due to -

Demographic window The period of time in a nation's demographic evolution when the proportion of population of working age group (15-55 years) is particularly prominent people < 15 years of age are less than 30% and > 65 years of age are less than 15%. The demographic window opportunity lasts for 30-40 years (i.e. from the age of 15 to age of 55). Demographic window of India is expected from 2010 to 2050 Demographic dividend A rise in the rate of economic growth due to a rising share of working age people in a population. It occurs when fertility rate falls. Lets see why :- Because of mechanical link between fertility levels and age structure, when the birth rate (fertility) falls, the age pyramid first shrinks with gradual lower proportion of young population (under 15 years) and proportion of population in working age group increases. This has following effects - i)  The respective nation has demographic window. Output per capita rises as population in working age group is prominent and dependency ratio (child dependency ratio) is decreased. In other words, decrease fertility rate is responsible for demographic dividend and demographic window. Demographic gift This the gift due to decreased fertility rate. That means favorable effects (demographic dividend) of decrease in fertility rate. Demographic transition A model used to explain the process of transition from high birth rate & death rate to low birth rate & death rate. Demographic trap This trap is the population growth due to high birth rate and low death rate. Demography bonus The term demography bonus connotes the period when the dependency ratio in a population declines because of decline in fertility, until it starts to rise again because of increasing longevity. Demographic burden It is the increase in the total dependency ratio during any period of time, mostly caused by increased old age dependency ratio.