Age-structure diagrams represent:

Age-structure diagrams, often referred to as population pyramids, visually represent the distribution of individuals across various age groups within a population. These diagrams provide insight into the characteristics of a population, such as its potential for growth or decline and the proportion of individuals at reproductive versus non-reproductive ages. By displaying these demographics, age-structure diagrams can help predict future population trends, such as the impacts of aging populations or youth bulges on a society's resources and economy.

Understanding the age distribution is crucial for planning in areas such as healthcare, education, and workforce management. This makes the correct option particularly relevant when analyzing demographic data for any given population. Other choices, while related to population studies, do not directly address what age-structure diagrams illustrate.