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==Effects on age structure==
[[File:Dtm pyramids.png|thumb|upright=1.4|One such visualization of this effect may be approximated by these hypothetical population pyramids.]]
The decline in death rate and birth rate that occurs during the demographic transition may transform the age structure. When the death rate declines during the second stage of the transition, the result is primarily an increase in the younger population. The reason being that when the death rate is high (stage one), the infant mortality rate is very high, often above 200 deaths per 1000 children born. When the death rate falls or improves, this may include lower infant mortality rate and increased child survival.
Over time, as individuals with increased survival rates age, there may also be an increase in the number of older children, teenagers, and young adults. This implies that there is an increase in the fertile population proportion which, with constant [[Total fertility rate|fertility rates]], may lead to an increase in the number of children born. This will further increase the growth of the child population. The second stage of the demographic transition, therefore, implies a rise in child dependency and creates a [[Population pyramid#Youth bulge|youth bulge]] in the population structure.<ref name=":0">{{Cite book|title=Population: An Introduction to Concepts and Issues|last=Weeks|first=John R.|publisher=Cengage Learning|year=2014|isbn=978-1-305-09450-5|pages=94–97}}</ref> As a population continues to move through the demographic transition into the third stage, fertility declines and the youth bulge prior to the decline ages out of child dependency into the working ages. This stage of the transition is often referred to as the golden age, and is typically when populations see the greatest advancements in living standards and economic development.<ref name=":0" /> However, further declines in both mortality and fertility will eventually result in an aging population, and a rise in the [[Age Dependency Ratio|aged dependency ratio]]. An increase of the aged dependency ratio often indicates that a population has reached below replacement levels of [[fertility]], and as result does not have enough people in the working ages to support the economy, and the growing dependent population.<ref name=":0" /> ==Historical studies==
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