Influence of age structure on population growth:
Even if the replacement-level fertility rate of 2.1 were magically achieved globally tomorrow, the world population would keep growing for at least another 50 years.
The reason for this is a population's age structure: the proportion of the population (or of each sex) at each age level.
Population Geographers typically construct a population age structure diagram by plotting the percentages of numbers of males and females in the total population in each of three age categories:
prereproductive (ages 0-14)
reproductive (ages 15-44)
postreproductive (ages 45 and up)
*** How Does Age Structure Affect Population Growth? ***
Any country with many people below 15 years old (represented by a wide based population structure diagram) has a powerful built-in momentum to increase its population size unless death rates rise sharply.
The number of births rises even if women have only one or two children because of the large number of girls who will be soon moving into their reproductive years.
In 2002, 30% of the world's people were under 15 years old.
These 1.9 billion people are poised to move into their reproductive years.
This powerful force for continued population growth, mostly in developing countries, could be slowed by:
*an effective program to reduce birth rates
* a sharp rise in death rates
Population size has stabilized or is declining in Japan and most European countries.
However, population size is expected to double or even triple before stabilizing for many developing countries with fairly large populations and large numbers of people under age 15.
Example countries are:
Nigeria
Ethiopia
Mexico
Brazil
Bangladesh
Pakistan
For example, 44% of Nigeria's population is under the age of 15 and its TFR is 5.8 children per woman.
As a result, Nigeria's current population of about 130 million is projected to reach about 305 million by 2050.
