This study investigated the developmental variability of lip and tongue movement in 48 children and adults. Motion of the tongue-tip, tongue-body and lower lip was recorded using electromagnetic articulography during productions of sentences containing /t/, /s/, /l/, /k/ and /p/. Four groups of speakers participated in the study: (1) aged 6-7 years; (2) 8-11 years; (3) 12-17 years; and (4) adults. The variation in distance, duration, speed, acceleration and deceleration of the articulators during single open-close speech movements was analysed, and the stability of multiple movement sequences was examined using the spatiotemporal index. The experimental findings revealed a gradual developmental progression from 6 years to adulthood. At adolescence, speakers continued to exhibit significantly more variable speech motor output compared to adult speakers. The observed developmental pattern suggests that attenuated, but important, changes in the speech motor system occurs from mid-childhood, through adolescence, to adulthood. (Contains 6 figures.)