In the expanding network of research on wearables, tracking technologies and ‘quantified selves’, aging bodies have been largely been neglected. At the same time, physical activity has been promoted as key to the prevention of many age-related problems – from falls to dementia – and inactivity had become framed as irresponsible. While research in the biomedical and exercise sciences focuses on how self-tracking devices can enhance interventions aimed at behavior modification with older adults, I argue we need to attend more carefully to the technologies, relationships and regimes of expertise that shape ageing experiences and subjectivities, and the ways in which quantification is embedded in everyday social worlds.