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In order to assess population aging objectively, several demographic indicators are usually proposed, including the average age of the population, the proportion of people aged 65 and over in the population, and the dependency ratio. A specific age is then used as a distinguishing criterion to define the population that is “no longer of working age.” This is a chronological concept of age. These indicators implicitly suppose that the characteristics of individuals (in particular, their state of health) are constant over time.Based on the assumption that increasing life expectancy means an increase in the number of years of life remaining, indicators of aging based on “prospective” age have been developed. Prospective age is defined as the number of years someone has remaining.The aim of this article is to highlight the value of the prospective approach from a theoretical point of view, and to give an illustration using an example in the area of health care, based on Belgian data.