Age-related differences in actual and perceived physical activity were investigated in a sample of adolescent girls, also considering the relationship between physical activity and motor abilities. Self-reported physical activity was measured in 11-year-old ( n = 66) and 13-year-old ( n = 52) girls by means of the Physical Activity Questionnaire–Children Form, and actual levels of activity were assessed by means of the Lifecorder uniaxial accelerometer. The Sit-and-Reach Test was employed to estimate flexibility, while lower limb strength was measured by squat jump and counter-movement jump tests. Weight and height were measured, and sex-specific age-related Body Mass Index cutoff points were used to identify overweight and obesity. Findings showed a drop in objective vigorous physical activity in older girls, which was also reflected in the Physical Activity Questionnaire. Contrary to expectations, no association was found between motor abilities and measured physical activity. Results highlight the need to understand the reasons why girls progressively reduce their involvement in physical activity as they get older, so as to plan interventions aimed at preventing this decline.