In demography, population aging refers to a change in the age structure of the population, from the structure of the old demographic regime prior to the demographic transition - with large groups of the population in childhood and youth ages and a small number of population in the mature and long-lived age groups - to a new structure typical of the second demographic transition or the so-called reproductive revolution - where basically the mature and long-lived age groups grow while maintaining or decreasing the younger groups. age.[1]​.

Population aging is characteristic of the second demographic transition and is a direct consequence of socioeconomic changes that allow greater generational survival—improved nutrition, hygienic and sanitary improvements, better care and attention, in short, a general increase in the quality of life.

The increase in life expectancy has grown significantly in many countries, producing, for the first time in human history, an abundance of mature and long-lived populations, a consequence of the success of public health policies and socioeconomic development. Part of the population that previously died in childbirth, in the first months of life, during childhood and youth and early adulthood, now survives for many years.[2]​.

Among the various studies that are carried out to analyze a specific population is that of age, which allows us to know the structure of said population. A visual method used to capture this analysis is the so-called “population pyramid”, a graph in which the most outstanding features of the various age groups can be detected.

The aging process of a society is usually a consequence of a low birth rate, produced by the containment of fertility typical of so-called advanced societies or developed countries, added to the growth of the elderly population, a process that in these societies is consolidated by the applications of improvements in health care and social services that allow greater survival of older people.

In developing countries, population aging is slowed by higher birth rates and higher mortality among the elderly population.

In January 2010, the United Nations published the report Population Aging 2009.[3]​

Four main conclusions stand out in the content of this report: