Variations in physical activity and aerobic power are examined in relation to age and gender of children and adolescents. Reviews of the world literature show aerobic power relative to body mass remains stable from ages 6 to 16 for males, but for females it declines about 2% per year. Overall, males are about 25% more fit than females. Reviews of nine studies using standardized self-reports or objective measures of physical activity revealed that males are about 15 to 25% more active than females. Over the school age years, a consistent decline in physical activity is seen, with males decreasing about 2.7% per year and females decreasing about 7.4% per year. These data suggest that older youth and females are at increased risk of obesity because of a sedentary lifestyle.