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Age and sex are among the most fundamental demographic characteristics of individuals. Viewed in the aggregate, age/sex composition forms the basic structure of human populations. It tells us the relative numbers of young and old as well as the balance of men and women at different ages. By characterizing the ”raw materials” of human populations, the age/sex structure indicates the numbers of people ”at risk” or ”available” to engage in a wide range of behaviors that vary by age (e.g., going to school, getting a job, committing a crime, getting married, starting a family, buying a home, getting divorced, retiring, getting sick and dying). By itself, it does not tell us who will engage in any of these behaviors, yet it does help determine overall patterns and trends.
Population aging is one of the most universal demographic trends characterizing early twenty-first-century populations. The age of a population simply refers to the relative numbers of people in different age groups. Populations around the world vary from being quite youthful (e.g., Uganda, where 51 percent of the population is under age 15 as of 2004), to being much older on average (e.g., Germany, where only 15 percent of the population is under age 15). The trend toward increasingly older populations is directly linked to declines in both fertility and mortality. With fewer births, the proportion of children declines, thereby raising the proportions at older ages; similarly, declines in adult mortality imply greater longevity and hence a larger proportion surviving to older ages. Trends in population aging are most evident in the more industrialized countries of Europe, North America, and Japan, where the percentage of the population over age 65 is projected to surpass 20 percent by 2030. However, a great many less developed countries can also anticipate rapid population aging in the near future as a result of their recent steep declines in both fertility and mortality.
The most common measure of the sex composition of a population is the sex ratio, which is simply the ratio of males to females (multiplied by 100). It is often assumed that populations are fairly balanced between men and women, but in most countries women outnumber men overall, though not necessarily at all ages. The sex ratio often declines with age because of progressively higher male than female mortality rates at older ages. In the USA, for example, the overall sex ratio is about 95 males for every 100 females; however, at birth, there are about 105 males for every 100 females, and by ages 85 and over, there are only about 40 males for every 100 females.
The dependency ratio is a summary measure of the age structure and is typically defined as the ratio of economically inactive to economically active persons. Since the economically inactive tend to be the young and the old, the dependency ratio is simply measured as the ratio of age groups (i.e. Children + Elderly/Working Ages). The precise ages used depends on the population being studied as well as the availability of data broken down by specific ages. In the USA for example, the dependency ratio is often measured as the ratio of ”persons under age 15 and over age 65” to ”persons of ages 15-64.” While it is recognized that many persons over age 15 are not yet economically active, and many persons over age 65 are still economically active, the dependency ratio approximates the number of inactive persons whom each active person must support. Given the different needs of children and elders, it is often useful to look separately at the child dependency ratio (Children/Working Ages) and the aged dependency ratio (Elderly/ Working Ages).
Data on age/sex structure are typically presented graphically in the form of an age pyramid, also known as a population pyramid. The pyramid can be thought of as two histograms placed on their sides and facing back to back, showing the age distributions for males on the left and females on the right. The vertical axis is age, coded in single years, or in 5-year age categories, with the youngest at the bottom. Each bar of the pyramid shows either the number or proportion of the population who are males or females in a given age group.
Since each bar is determined by past demographic patterns, it follows that the overall shape of the pyramid does as well. Rapidly growing populations, in which births far exceed deaths, are typically characterized by a wide base and a classic ”pyramid-like” shape (i.e., each new cohort is larger than the previous one). In contrast, a population which is neither growing nor declining has a more rectangular shape whereby each new cohort entering at the bottom is roughly the same size as the preceding cohort. A population which is declining due to an excess of deaths over births would have an age pyramid which is narrower at the base than at older ages.
- Rowland, T. (2003) Demographic Methods and Concepts. Oxford University Press, Oxford.