We investigated the possibility that marked improvements in speed of information processing from early childhood to adulthood reflect improved speed-accuracy monitoring and regulation. Trial-by-trial examination of RT and accuracy transitions during serial choice RT performance revealed developmental changes in accuracy monitoring and speed regulation; these changes corresponded to the most pronounced age-related changes in average RT. For 5-year-olds, poor control over speed of responding, coupled with inconsistent accuracy monitoring, resulted in less orderly trial-to-trial RT transitions and a consequent failure to constrain responses within fast RT bands just safely above overly fast error RT levels. Control over speed of responding was improved by age 7, but inconsistent accuracy monitoring was still a factor. From age 9 up to adulthood, subjects monitored accuracy consistently and showed quite precise control over speed of responding. These developments were associated with a marked improvement in RT constraint within narrow, fast RT bands.