Why are population pyramids sometimes called age structure diagrams

A population pyramid consists of a vertical number line, but instead of numbers its ages. The diagram shows how much of the population is in each age group, and also divides it again to show if the person is a boy or a girl. These pyramids/diagrams help compare the ages and genders of a population in an easy way. (Ex. Large amount in the youth but small amount in the elderly would mean economic problems, Ex. Many boys on one side but not many girls in reproductive age would mean population will decline)