Mandatory retirement because of chronological age is coming under increasing attack and, at least in the United States, it is likely that there may soon be legislative prohibitions against forcing individuals to retire because of age. As a consequence there is renewed interest in redefining retirement criteria in terms of a functional age concept which would give due attention to individual variability in the maintenance and development of behavioral competence. This paper calls attention to a number of problems with the undifferentiated use of the functional age concept and indicates a number of possible alternative approaches, some of which may be more appropriate for application to retirement issues than others. A distinction is made between functional age as a general index applicable to basic psychophysiological mechanisms and a matrix of functional ages referring to specific social or environmental situations. (Author)