Abstract
Claims of accelerated or premature aging are frequently made. However, the lack of standard criteria for measuring speed of aging makes such claims highly questionable. Because of fundamental gaps in our current understanding of the biological mechanisms of aging, the development of specific phenotypes that are due to aging is difficult and such phenotypes can only be derived by observational data. However, a clinical phenotype of aging exists that is experienced by all living individuals and is pervasive across multiple physiologic systems. Characterizing this phenotype can serve as a basis for measuring the speed of aging, and can facilitate a better understanding of the aging process and its interaction with chronic diseases.