The demand and requirements for valid, practicable, and reliable procedures for age diagnosis are increasing worldwide. In contrast, few studies and only a small number of procedures exist. The authors review the theoretical and methodological requirements for the development of models for age diagnostics. They describe the fundamentals for further studies, based on an analysis of current gerontological research in this area. A following publication will report the valid systems measuring vitality and biofunctional age(ing) of human beings.