The main theories on population aging based on recent data on human longevity, life expectancy, morbidity changes, disability trends, and mortality decrease are presented and discussed within their own geographic, cultural, socioeconomic, and medical contexts. The complex interactions between all these components do not facilitate trend forecasting of aging population (healthy aging versus disability pandemic). In the context of population aging, four elements were introduced with their implications: 1) an increase in the survival rates of sick persons, which would explain the expansion of morbidity, 2) a control of the progression of chronic diseases, which would explain a subtle equilibrium between the decrease in mortality and the increase in disability, 3) an improvement of the health status and health behaviors of new cohorts of elderly people, which would explain the compression of morbidity, and eventually 4) an emergence of very old and frail populations, which would explain a new expansion of morbidity. Obviously, all these elements coexist today, and future trend scenarios—expansion or compression of disability—depend on their respective weights leading to the need of elaborating "a general theory on population aging.” This theory has to be based on a world harmonization of functional decline measurements and a periodic "International Aging Survey” to monitor global aging through a sample of carefully selected countrie
