Associated movements (AMs) are the most frequently assessed parameters of movement quality in children with motor dysfunctions. In this article, reference curves of duration and degree of AMs from 5 to 18 years are provided. In a cross-sectional study of non-disabled children (n=662) duration and degree of AMs were estimated at six specific ages while children performed repetitive finger, hand, and foot movements, alternating hand and foot movements, diadochokinesis, sequential finger movements, pegboard, stress gaits, and dynamic balance. Moderate-to-high intraobserver and interobserver reliability for the assessment of AMs were noted. Duration and degree of AMs displayed a non-linear developmental course that was a function of the motor task's complexity. AMs decreased most with age in repetitive movements, less in alternating and sequential movements, and least in the pegboard and dynamic balance. Reference curves demonstrated large interindividual variations for duration and degree of AMs. Both the variable developmental course and large interindividual variation need to be taken into account in the assessment of movement quality of school-age children. In contrast to timed performance, considerable sex differences for AMs were observed.