We tested whether the relationship between subjective physical age and physical activity is mediated by planning. Participants came from a broad age range (25–78 years, M = 39.57, standard deviation = 10.75) and reported relatively good health (M = 3.36, standard deviation = 0.90). The model supported the suggested mediation (β = –.01, standard error = .01, p = .042). Feeling physically younger is associated with higher planning to adopt higher levels of physical activity and more planning is associated with more subsequent physical activity. Results open avenues for interventions that help people to become more active by focusing on subjective age. One way to do so might be tailoring approaches for interventions.