As the population ages and life expectancy increases, it is crucial to develop a definition of aging that can effectively guide daily clinical decision-making and, most importantly, shape the development priorities of our health system to address the growing needs for healthy life expectancy. The definition of aging solely based on chronological age leads directly to an “ageism” process. In addition, it cannot serve as a basis to meet medical needs due to the extreme heterogeneity observed among an old population of same age. Another approach is to define aging from a biological, bio-genetical or clinical perspective, which is often difficult to operationalize. In addition, highly prevalent comorbidities are suboptimal predictors in terms of mortality. In a perspective of healthy aging, the functional approach to define age seems to be the most adequate. The importance of the functionality of older persons, as a predictive factor of mortality and adverse clinical outcomes is the focus of the thesis, as well as its assessment using easily accessible clinical scales. Functional age, as a new and innovative definition of healthy aging, could be used as a basis for developing public health programs.