The composition of population according to age and/or sex is known as the age and sex structure. The universal characteristics of human populations are fundamental to understanding demographic processes of fertility, mortality and migration.
Age composition may be summarized in terms of age groups (e.g., 0-15 years, 15-64 years and 65 or above).
In India, the age groups are below 15, 15-59 and above 59, as the age of retirement in our country is generally 60 years. The sex ratio is most commonly expressed as the number of females per 1,000 males.
One of the important aspects of the population study is the age composition. The age composition strongly influences the rate of growth and has profound effects on the social and economic conditions under which a population lives.
Three basic determinants of age composition are: (i) nasality, (ii) mortality, and (iii) mobility. These are interdependent, and any change in one may influence the other two.
It is through these variables that the socio-economic conditions influence the age structure. It is the fertility rate that determines the proportion of population in different age categories.
That is why; most of the countries in Asia (excluding Japan, Singapore, Hong Kong and South Korea), Latin America and Africa have high fertility. Since the longevity and life expectancy in these countries is short, the proportion of population in the old age group is also not very large.
According to the Census of 2010, in most of the countries of Europe and North America, about 20 per cent of their total population is below the age of 15 years.
In India, about 33 per cent of its population is below 15 years in age and 5.5 per cent above 60 years. On the contrary, the European countries, with low fertility rate and long life expectancy, have only 20 per cent of their populations in the younger age group.
Moreover, the proportion of population in the older age groups in developed countries is relatively large.
Consequently, most of the countries of Europe, Anglo-America, Japan, Australia and New Zealand, which have almost completed their demographic transition, have an age composition in which the proportion of young population (15) is low and that of the old population (65) is high.
The age composition is also affected by the rate of mortality. In general, if the survival rate of the childhood improves, the proportion of children will tend to rise and that of the older people will tend to fall.
And, if the survival rate improvement takes place only among the older people, the proportion of older people will improve but that of children will tend to fall.
Similarly, if the mortality is low both among younger and older age groups as in the case with developed countries like Germany, Italy, Sweden, Netherlands, Belgium and Denmark, the share of the working population will be larger and the dependency ratio will be low.
In contrast to this, if the decline in mortality is sharper in lower age group than that in the older age group, it results in the swelling up of numbers in younger age group as is the case with most of the populations of developing countries.
Migration also has a direct bearing on the age composition. Those who migrate, generally belong to the relatively younger age group.
When the youths out migrate, the population of older adults and aged people starts declining after some time. The impact of migration upon the age structure is largely because of the fact that migration tends to be age-selective.
Generally, people in the age group of 15-30 years are more mobile than the people in the younger and older age groups. Consequently, the population of the juvenile and senile age groups increases at the place of origin of migration.
Apart from fertility, mortality and migration, the age structure of population is also influenced significantly by wars (male-selective in their casualty), catastrophes, natural calamities and population policies.
The age structure of a given country or region may be analyzed on the basis age groups.
On the basis of physiological and economic activities, the population is generally classified into three groups: (i) the young, (ii) the adults, and (iii) the old.
Although there are no standardized break points, yet the ages of 15 and 60 years are the most commonly used break points in the developing countries and 15 and 65 years in the developed countries.
The social, economic and political implications of these age groups and the geographical variation in their distribution are worthy of serious consideration.
I. The Young:
All over the world, the young age group includes the population below 15 years of age. The proportion of population in this age group in any country is determined by the stage of demographic transition it is passing through.
This proportion is large if the country is passing through the first or the second stage of demographic transition. It starts declining as the country approaches the late stage. It is the minimum when the country is in the final stage of demographic transition.
It is interesting to note that while in the world as a whole about 30 per cent population is below the age of 15 years, the corresponding figures for the developed and the developing regions are 20 per cent and 36 per cent respectively. In Germany, the proportion of young population (below 15 years) is only 15 per cent.
In Italy, UK and France the adult population is only 14 per cent, 18 per cent and 17 per cent (2001) respectively.
Thus, there are wide regional variations in the proportion of young population ranging from less than 18 per cent in Europe to nearly 43 per cent in Africa and about 36 per cent in Asia and Latin America. In India, about 33 per cent of the total population is below 15 years of age.
These regional variations are related with the fertility patterns of different countries. Countries that are characterized with high fertility rates have large proportions of young populations and vice versa.
There seems to be a direct correlation between birth rates and proportion of young population. This age group is economically unproductive and the most expensive as it is to be provided with food, clothing, education recreational, health and medical facilities.
II. The Adults:
The group of adults consists of 15 to 65 years in the developed and 15 to 60 in the developing countries.
Depending upon the life expectancy, the upper limit in the developing countries is 60 and that in the developed countries is 65. The adult age group is biologically the most reproductive, economically the most productive and demographically the most mobile (migratory).
It supports the bulk of other age groups; it carries the burden of feeding, clothing, educating the young age group and of looking after the old age group. The developed countries have relatively high proportion of adult population.
This adult age group is sometimes divided further into two sub-groups: (i) 16 to 34, and (ii) 35 to 59/64. The first sub-group of young adults is considered biologically most reproductive and economically more active than the second one of older adults
III. The Old:
The old age group consists of those who have achieved the age of 60 years in the developing countries and 65 or above in the developed countries. Such people are called as the senior citizens. The proportion of people in this age group, by and large, increases as the population of a country completes its demographic evolution.
In the developed countries, the number of females in this age group is more than that of the males because the proportion of population in this age group is governed largely by mortality rates, and since the male mortality rate in the developed countries is higher than that of the females at all ages, the females tend to outnumber the males in this age group.
Biologically, the males belonging to this age group usually remain less reproductive. This age group is largely an economic burden upon the family as it is to be provided with food, clothing and adequate health care under the social security system.