The relationship between age and the level of performance of major league baseball players was assessed through quasi-experimental designs. Whereas cross-sectional comparisons revealed no differences in batting and fielding statistics between younger and older players, longitudinal analysis showed significant decrements in batting performance as players aged from 30 to 35 years. A decline in performance with age was found even among elite players. Age decrements in achievement need to be studied not only in the context of molar measures such as batting statistics but also at a microanalytic level through reference to component skills. This paper outlines a methodology that can be used in assessing the nature and basis of age decrements in skilled athletic performance.