Age and sex are among the most fundamental demographic characteristics of individuals. Viewed in the aggregate, age/sex composition forms the basic structure of human populations. It tells us the relative numbers of young and old as well as the balance of men and women at different ages. By characterizing the “raw materials” of human populations, the age/sex structure indicates the numbers of people “at risk” or “available” to engage in a wide range of behaviors that vary by age (e.g., going to school, getting a job, committing a crime, getting married, starting a family, buying a home, getting divorced, retiring, getting sick and dying). By itself, it does not tell us who will engage in any of these behaviors, yet it does help determine overall patterns and trends.