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Physical activity (PA) decreases from childhood to adolescence, with girls being less active than boys. The timing of these differences remains unknown. Using accelerometer data from three cross-sectional studies in Norway and Belgium (n = 2507, age = 3-17years), we assessed sex differences in sedentary behaviour (SB) and PA levels (light, moderate, vigorous) throughout the day and across the full spectrum of activity intensity distribution on weekdays and weekend days, using linear regression and functional data analyses. Across all age groups (preschoolers (3-5y), children (6-10y), adolescents (11-17y)), girls were less active than boys, particularly on weekdays (e.g., vigorous PA (> 1111 counts/15s) difference:-16.9 min/day (95% Confidence interval:-19.3,-14.4; p-value < 0.001) in children). It was the case throughout the day, particularly during school hours (8h30-15h29) in all age groups. Analysis of the full spectrum of activity intensity distribution (0 to 3000 counts) added to these findings that on weekend days, girls spent less time in zero-count SB than boys (difference=-21.0 min/day (-28.7,-13.4; p-value < 0.001) in children), but higher (17.3 min/day (13.2,21.4; p-value < 0.001)) in the “other SB”, 1-180 counts/15s. The sex differences in PA during school hours suggest the need for targeted interventions promoting activities engaging girls. Additionally, the time spent in zero-count, particularly evident in boys on weekend days, deserves further investigation.