A large base and steeply tapering sides (a real pyramid form) indicate high fertility and death rates in younger age groups. Irregularities in the profile of the population pyramid transmit information about population shifts or aberrations. For example, if the population is mainly made up of young people but then starts to decline rapidly, this would be indicated by a strongly sloping pyramid with a narrow base. An older population supported by a strong economy and good health services could produce a population pyramid with a wide base.
Information about mortality can also be obtained from study of the burial ground. A high rate of infant mortality indicates that many children are dying before they reach adulthood. This may be due to disease or accident but it also could be due to lack of nutrition or care during pregnancy or after birth. If most adults were still buried after only a few years instead of decades as expected, this would show that there were not enough old people to take care of the dead.
Finally, information about fertility rates can be obtained from studying the size of the base of the population pyramid. If the number of children born per family stays constant over time, then the total population will increase if any deaths exceed births. If on the other hand, more deaths than births occur, then the population will shrink.
The form of pyramids indicates the population's growth rate. The population with a high proportion of youthful members increases swiftly, whereas the population with a high proportion of post-reproductive members tends to fall. This sort of age pyramid indicates that the population is increasing. Conversely, a pyramid shaped like a point or a knife indicates that the population is declining.
Around the world, most populations are aging and becoming less fertile. As a result, their age pyramids tend to be skewed toward the older end: often more than half the population is over 50. A few populations, including some Native Americans, have age pyramids that are nearly flat, indicating they have relatively stable numbers even as they get older. Others, such as many African countries, will experience a rapid increase in their number of elderly residents.
These trends will likely continue around the world for the next 100 years or so. In fact, recent data show that more than 40 percent of the world's population now lives in an area where the fraction of people aged 15-49 is below 5 percent.
The share of the world's population that is younger than 30 has dropped from 27 percent in 1970 to 22 percent today. At the same time, the percentage of people aged 60 and above has more than doubled, from 14 percent to 33 percent.
They are distinguished by their rectangular form, with relatively comparable percentages throughout age cohorts that taper off toward the top. These pyramids are common in industrialized countries with low birth rates and excellent overall quality of life. They also show up to some extent in most developing countries with large populations and limited resources.
In well-off countries, the young are usually not poor nor are the old necessarily rich. Rather, there is generally enough wealth distributed among the population as a whole so that everyone can enjoy good living standards regardless of their age. A society where this is true has no elderly poverty or youth unemployment.
In contrast, a predominantly older population will be made up of people who were either unable to find work or who chose not to work after the age of 50. Such societies often have high rates of unemployment and poverty especially among older people.
A pattern of increasing prosperity with aging is called an "aging wave". These waves occur when new technologies create more jobs than old ones, so that even though there are more people over 50 than under 12, the total workforce remains about the same. This allows for more leisure time and more investments in education and health care resulting in greater productivity and growth over time.
An example is provided by Japan which has had an aging population for several decades now.