The probability of dying or giving birth often varies with age, especially for long- lived species such as humans. Age-related changes in mortality and fertility need to be taken into account in order to predict how these populations will change over time.
Cohorts are subsets of populations composed of individuals of similar age.
Demographers project how age-structured populations change over time by following survival of cohorts from age category to age category, and recruitment of newborns from reproduction in each age cohort. The distribution of individuals in each age category defines the age structure of a population. Rapidly growing human populations have a high proportion of individuals in young age classes. This leads to a population momentum, as these young mature into reproductive age classes and lead to future increases in the absolute number of births in the population. The demographic transition describes the decreases in death rates and birth rates due to the change from traditional agarian societies to industrial societies.