The influence of age and fitness on the neuroelectric correlates of attentional orienting and processing during stimulus discrimination were investigated. Younger and older adult participants completed a maximal aerobic exercise test and were separated into higher- and lower-fit groups according to their cardiorespiratory fitness. Task performance and event-related potential measures were obtained during two- and three-stimulus oddball tasks. Results indicated that fitness may ameliorate or protect against cognitive aging for simple stimulus discriminations. Increases in task difficulty indicated that fitness may not be sufficient to overcome age-related deficits in stimulus discrimination. Further, fitness did not influence attentional orienting. The findings suggest that fitness-related changes in cognitive function may originate from other attentional mechanisms. Theoretical implications are discussed.