PURPOSE: Activity patterns in children tend to be intermittent and variable in nature, particularly during free play and sport; the purpose of this study was to examine the effects of carbohydrate (CHO) supplementation in children during this type of activity. METHODS: Boys (n = 5, 11.3 ± 1.1 yrs) and men (n = 7, 23.8 ± 2.6 yrs) of similar aerobic fitness (VO2peak = 44.5 ± 6.5 and 46.3 ± 3.7 ml/kg/min for boys and men, respectively) consumed 1.5g/kg of CHO or a placebo (PL) beverage before and throughout a variable-intensity exercise protocol (VIE). VIE included three 12-minute sets of cycling; intensity varied every 20-30 seconds between 25 and 125% VO2peak. Blood glucose and lactate concentrations were measured before and every 12 minutes during exercise. VO2 and respiratory exchange ratio (RER) were measured during the second 12-minute set of VIE and perceived exertion (RPE) was recorded every 6 minutes throughout exercise. A three-way (group × time × trial) ANOVA was used to analyze the data. RESULTS: VO2 expressed as a percentage of VO2peak was similar between boys and men during VIE and was not affected by CHO. In CHO, men had higher pre-exercise blood glucose concentrations than boys (124 ± 17 vs. 96 ± 10 mg/dL, p < 0.05). In PL, men had higher post-exercise blood glucose than boys (94 ± 8 vs. 82 ± 5 mg/dL, p < 0.05). In boys, post-exercise blood glucose was lower during the PL trial compared to the CHO trial (96 vs. 82 mg/dL, p < 0.05). RER was significantly lower in boys than men (0.96 vs. 0.99) and increased over time (p < 0.05), but was not significantly affected by CHO supplementation. Men had significantly higher lactate than boys throughout VIE. In men, lactate was significantly elevated at all exercise time points, while in boys lactate was only significantly increased after the first 12-min set. RPE increased over time during VIE (p < 0.05), but did not differ between boys and men and was not affected by CHO supplementation. CONCLUSIONS: Some physiological responses to VIE varied between boys and men. However, CHO supplementation prior to and during VIE does not appear to provide a physiological or perceptual benefit to either boys or men during this type of exercise.