This paper presents a model of cognitive development across the life span that is designed to integrate the diverse empirical evidence in the area of cognitive development in childhood and adulthood. Relevant research is reviewed in the relationships between performance and age, and the effects of training on performance. A model is then proposed which: (1) draws a distinction between an individual's unexercised ability level (the level of performance that would be expected if the individual had no exercise and/or training on the ability in question) and an individual's optimally-exercised ability level (the level of performance that would be expected if the individual had optimal exercise and training); (2) suggests that both unexercised and optimally-exercised ability levels increase with age up to early adulthood and decrease gradually thereafter; and (3) suggests that the region between the two ability levels also increases during childhood and decreases during later adulthood, thereby indicating the extent to which exercise and/or training can effect performance. This model is shown to be consistent with different types of empirical findings. Additionally, limitations of the model are discussed. (NRB)