Critical reviews of the literature on the factors that influence physical activity among older adults have argued that existing theoretical frameworks should be extended by integrating those that deal with the complex processes of socialisation and social learning. This paper explores some of the social processes that influence older people's participation in physical activity (beyond that associated with everyday domestic tasks). A questionnaire with items on personal, social and environmental characteristics was completed by a random sample of older adults in the Madrid Autonomous Region (Spain). Significant relationships were found between the type of physical activity participation and: being physically active at earlier life stages, socio-economic status, the encouragement of others or social support in being active, and the knowledge and availability of local facilities. Some cases were observed of re-socialisation into physical activity among those who had been inactive earlier in life, and both appropriate environmental and supportive social conditions appeared instrumental. The findings could usefully inform the design of future social programmes to promote active lifestyles in later life, but given the complexity of the socialisation processes, it would be advisable for future studies to examine other than the four factors featured in the presented analysis, such as the role of cultural differences.