Children ages 4, 6, and 8 years and adults performed self-selected, continuous, unimanual and bimanual coordination tasks for 30 s. The length of time performing the task was investigated as a potential control parameter. As hypothesized, all groups spent less time in antiphase than in in-phase coordination as the trial continued. These results were interpreted as evidence that the length of time performing a task is a control parameter embedded in any task. The importance of studying control parameters in various developing systems is discussed. (Contains 3 figures.)