To investigate changes in time spent in light (LPA), moderate (MPA), vigorous (VPA), very high physical activity (VHPA) from childhood to adolescence, according to age and sex, when measured with high frequency accelerometry. Three hundred and sixty-one children, 94 preschoolers (Ps), 156 from primary schools (PS) and 111 from junior high schools (JHS)) were involved in this study. The children's physical activity was assessed with a uniaxial accelerometer over a seven-day period. The epoch duration was set at 5 s and data collected between 7 am and 9 pm. The times spent below and above different PA thresholds, corresponding from LPA (9 METs), were calculated. During the week, the boys spent significantly more time in MPA to VHPA than the girls (p < 0.001). From Ps to PS, LPA remained stable, while VPA and VHPA decreased significantly (p < 0.05). From PS to JHS, time spent in LPA, VPA, and VHPA increased significantly (p < 0.05). On the contrary, MPA increased significantly (p < 0.05) between Ps and PS and decreased significantly (p < 0.05) from PS to JHS. From PS to JHS, time spent in LPA increased significantly more during free days than during school days (p < 0.05) while VPA and VHPA increased significantly (p < 0.05) more during school days than during free days. Moderate to very high PA decreased from childhood to adolescence. Changes in PA patterns were associated with an increase of LPA and a concomitant decrease of MPA, while changes were more pronounced during free days than during school days.