This study examines age-differential association patterns between intentions, planning and physical activity in young and middle-aged individuals. The effectiveness of planning to bridge the intention-behaviour gap is assumed to increase with advancing age. We explore the use of behaviour change strategies that include selection, optimisation and compensation (SOC) as underlying mechanisms for age differences.In N = 265 employees of a national railway company (aged 19-64 years), intentions, planning, SOC strategy use and physical activity were assessed at baseline (Time 1) and again 1 month later (Time 2). Hypotheses were tested in two different path models.Age moderates the extent to which planning mediates the intention-behaviour relation due to an increasing strength of the planning-behaviour link. As a possible psychological mechanism for these age differences, we identified SOC strategy use as a mediator of the age by planning interaction effect on physical activity.These findings suggest differential mechanisms in behaviour regulation in young and middle-aged individuals.