An age structure diagram is a model that predicts the population growth rate by a shape. It shows a comparative ratio of males to females and the bars show various age groups from infants - adolescents - reproduce - post reproductive.
Pre reproductive age 0-14
By using an age structure diagram it can be predicted whether a population will increase, decrease or maintain stability. Predictions can also be made as to why this trend is showing for example: is the population decreasing because most of the population is in the post productive age group?
The image below shows an age structure diagram for Ethiopia, and a classic example of a population in rapid growth. There are many more children in each generation than their parents, meaning that couples are having a lot of children. In this example the majority of the population is very young.
The diagram below shows New Zealand and an example of a population with stable numbers. This population has about the same amount of individuals in each age bracket. This suggests parents have access to reproductive education and can choose replacement level reproduction, or to have about two children per each couple.