Inhibitory control functions in old age were investigated with the "masked prime" paradigm in which participants executed speeded manual choice responses to simple visual targets. These were preceded--either immediately or at some earlier time--by a backward-masked prime. Young adults produced positive compatibility effects (PCEs)--faster and more accurate responses for matching than for nonmatching prime-target pairs--when prime and target immediately followed each other, and the reverse effect (negative compatibility effect, NCE) for targets that followed the prime after a short interval. Older adults produced similar PCEs to young adults, indicating intact low-level motor activation, but failed to produce normal NCEs even with longer delays (Experiment 1), increased opportunity for prime processing (Experiment 2), and prolonged learning (Experiment 3). However, a fine-grained analysis of each individual's time course of masked priming effects revealed NCEs in the majority of older adults, of the same magnitude as those of young adults. These were significantly delayed (even more than expected on the basis of general slowing), indicating a disproportionate impairment of low-level inhibitory motor control in old age.