The purpose of this study was to examine the influence of child and adult cut-points on physical activity (PA) intensity, the prevalence of meeting PA guidelines, and association with selected health outcomes. Participants (6,578 adults > or = 18 years, and 3,174 children and adolescents < or = 17 years) from the National Health and Nutrition Examination Survey 2003-06 (Centers for Disease Control and Prevention, 2006) wore an accelerometer for 7 days. PA intensity was estimated with 5 child-derived and 12 adult-derived cut-points. For all, the cut-point influenced PA intensity and the prevalence of meeting PA guidelines. Similarly, cut-point selection influenced the relationship between physical activity and various health outcomes. Future research should further enhance meaningful cut-points relevant to populations with diverse health and age profiles.