PURPOSE This study examines the differential age effects on physical exercise of two planning interventions, action planning (when, where, how) and coping planning (anticipating barriers, mental simulation of success scenarios), and examines the mediating mechanisms of the interventions. METHODS The study assigned the participants, 205 cardiac rehabilitation patients, to one of the intervention groups (action-planning only or combined-planning group) or to a control group. Baseline measurement and follow-up took place 2 months apart. RESULTS The interventions enhanced physical exercise independently of age. Pretreatment coping planning was higher in older (65-82 years) than in younger (38-54 years) or middle-aged (55-64 years) participants. At Time 2, older participants were the only ones without further increase in coping planning. Advancement in coping planning partially mediated the effect of the intervention. CONCLUSION Coping planning facilitates improvement of physical exercise. Implications of age differences in planning are discussed.