i. Age structure is an expression of the number of people in a total population found in each age group.
ii. Generally the population is categorised into three broad age groups: the young (usually under 14), adults (usually 15 to 59 inclusive) and the aged (60 and over).
iii. It is said that age structure reflects the demographic and socio-economic history of a population over a period of about a century. Certainly it is the result of various and interrelated factors such as fertility, mortality and migration, which have operated during the lifetime of the oldest inhabitants.
iv. Age structures of particular countries are most commonly shown diagrammatically, using age-sex pyramids. Four main types have been identified:
1. A progressive age structure of population is one in which both birth and death rates are high.
i. Children = 45-55% and aged = 5-10% of the total population
ii. High fertility due to social, cultural and perhaps religious and economic condition. High level of mortality due to poor living conditions, bad diets and little medical aid,
iii. Developing countries such as Bolivia and Angola.
iv. Pyramid shape – Typical of first stage of demographic transition with very broad base but rapidly tapering.
2. A regressive age structure of population is one in which birth and death rates are low and declining.
i. Children = 30% and aged = above 15% of the total population.
ii. Regions of high living standards, education and social awareness accompanied by good food and medicine. Ex-developed countries (especially those of Western Europe)
iii. Pyramid shape – Narrower base with equally wide subsequent age groups tapering off gradually.
3. A stationary age structure of population is one in which birth and death rate unchanged over a long period of time.
i. Children = 35-40% and aged – about 10%, of the total population. Ex-Japan.
ii. Pyramid shape – Regularly tapering.
4. An intermediate age structure may vary in character and is most common in countries that are passing through stages of development ex- USA. Such countries may once have had progressive structures and may, in future, have regressive structures.