The purpose of the present study was to compare body composition and physical activity level between children and adults with Autism Spectrum Disorders (ASD). A sample of 78 children, adolescents and adults participated in the study. Anthropometrics and physical activity, using GT1M accelerometer, were assessed. Overweight and obesity prevalence was higher in men vs. male children (p < 0.001) and in men vs. women (p = 0.035). Children recorded more moderate to vigorous physical activity (p = 0.040) than adults. Normal-weight children and adolescents combined as one age group, accomplished more moderate to vigorous physical activity, steps and less sedentary time compared to their overweight and obese counterparts during the weekend. Obesity status may negatively affect physical activity level in ASD individuals.