3 experiments were conducted to study developmental change in the speed of cognitive processes. In Experiment 1, subjects ranging in age from 8 to 21 years were tested on a mental rotation task in which they judged whether pairs of letters presented at different orientations were identical or mirror images, and on a name retrieval task in which they judged if pairs of pictures were identical physically or had the same name. Increases with age in the speed of mental rotation and speed of name retrieval were both well described by exponential functions, and the rate of developmental change was comparable for the 2 tasks. Experiment 2 confirmed that age changes in rate of mental rotation are well described by an exponential function. Experiment 3 demonstrated, on a mental rotation task, a perfect correlation across conditions between children's mean response times and adults' mean response times for corresponding conditions. These results are interpreted as indicating that age differences in speed of processing are due, at least in part, to a central limiting mechanism that increases with age.