The worst performance rule (WPR) predicts that the slowest trials in reaction time (RT) tasks are more strongly related to intelligence than the fastest trials. To date, the WPR was observed mainly in young adults. The present study examined if the WPR holds not only in young adults but also in children and older adults in three kinds of RT tasks (simple RT, choice RT, and inhibition). Results showed that in each age group slowest and fastest trials were related to intelligence but the former correlated with intelligence to a greater extent than the latter. These results support the assumption that the WPR can be generalized across the lifespan.