Although both physical fitness and aspects of cognitive functioning are known to be poorer among older people, there is conflicting evidence about the interaction between age and fitness. Is greater age more strongly associated with impaired cognition among less fit people than it is among more fit people? In a sample of employed men aged 17-63 years, it is shown that this interaction is significant with respect to the occurrence of exceptionally slow responses but is not significant for mean reaction time. Multiple regression analyses suggest that the former measure may be interpreted as an inability to maintain concentration over time rather than as an index of response speed.