The ability to move two fingers at the same time was measured in young and older adults. This study measured the accuracy with which 45 normally developing children (23 boys, 22 girls; mean ages 8.4 and 8.3 years respectively) and 49 young adults (25 males, 24 females; mean ages 19.8 and 20.5 years respectively) could synchronize movements of two fingers when making or breaking contact with circular metal discs. A portable electronic instrument displayed the timings. To measure 'in-phase' skills, attempts were made to contact both discs simultaneously with the two index fingers or to break both contacts; asynchrony was only a few milliseconds and differences between children and adults were insignificant. To measure 'antiphase' skills, attempts were made with the index and middle fingers to make contact simultaneously with one disc while breaking contact with the other; asynchrony was larger. Usually one contact was made and later the other was broken; for a while both fingers touched (overlap). Rarely one contact was broken before the other was made; for a while neither finger touched (gap). Boys' periods of overlap were longer than those of men and women; on both sides these differences were highly significant statistically (p<0.0001). Boys' errors on the right side were also significantly greater than those of girls. On both sides, mean errors of girls were higher than those of men and women: three of the four sets of data reached statistical significance. While boys' performance improved with age, that of girls was static. Children achieved an essentially adult level of control for in-phase skills but with antiphase skills the children, and especially the boys, had clearly poorer performances.