[Can age and aging be measured?]

Are age and aging measurable? Yes, of course they are, if all types of quantification are included. Nevertheless, a lot of questions have to be discussed with regard to different concepts of age, e.g., chronological age, biological age, subjective age, mean age, life expectancy, etc. Even the question when someone should be labeled old cannot be answered without reference to a priori hypotheses and demographic development. Beyond that, aging is not a standardized process. Aging is dependent on multiple individual and environmental conditions and there is a large interindividual variability which is often underestimated. Research on these aging processes is itself dependent on the instruments used, e.g., the methods of a longitudinal or cross-sectional study. Aging is a multifaceted and quite individual process.