Many children and older adults do not meet WHO physical activity (PA) guidelines. Intergenerational programs may help. Ages Get Active is a theory-based program co-developed to promote joint PA between grandparents and grandchildren through group, home, and community activities. This study evaluated its effectiveness and implementation using a mixed-methods ABAB design. Over 12 weeks, PA was measured with accelerometers, co-PA with calendars, and well-being with biweekly questionnaires. Semi-structured interviews explored relationships. Process evaluation used feedback forms, interviews, and a focus group. Nine grandparents, nine grandchildren, and eight parents participated. Grandparents generally increased PA during intervention phases, with small-to-moderate co-PA gains; one dyad showed a large effect. Children's PA changes were inconsistent. Grandparents reported higher well-being (p = .006). Qualitative data indicated stronger bonds. Group sessions were valued; home/community booklets less. Motivation and sustaining home activities were challenging. Ages Get Active shows promise for enhancing co-PA, well-being, and intergenerational relationships.
