Effects of prior-task failure (i.e., decreased performance on a target task following failure on a prior task) were tested in young and older adults. Young and older participants (N=120) accomplished a computational estimation task (i.e., providing the best estimates to arithmetic problems) before and after accomplishing a dot comparison task in a control or in a failure condition. Both groups decreased their performance on the target computational estimation following failure on the prior dot comparison task. Also, prior-task failure led young and older adults to select the better strategy less often and to use the easier strategy more often. Our findings show, for the first time, impaired performance after experiencing failure in both young and older adults. We discuss implications of these findings for further our understanding of effects of task transitions (i.e., prior-task success and failure) on cognitive performance.