Background: Self-regulatory behaviour change techniques (BCTs) appear less effective in promoting physical activity amongst older than younger adults. This study aimed to identify reasons for this by examining how participants of different ages understand, experience, and use these BCTs. Methods: Twelve participants (aged 39–75) in a walking intervention study were interviewed twice: immediately post-intervention and 3 months later to examine understanding and enactment of self-regulation BCTs. Thematic analysis was used, organised using the framework approach. Results: Participants acknowledged the importance of setting realistic goals and found pedometers useful. In older adults, the use of goal setting was influenced by previous experience in work settings of this BCT. Occupational status appeared to influence the participants' responses to action planning, irrespective of age, with retired participants preferring not to restrict themselves to specific times. Self-monitoring with diaries appeared to be more useful in assisting the memory of older adults. For most BCTs, differences in understanding and enactment were apparent according to participant age. Conclusions: Problems with using self-regulation BCTs were apparent, which appeared more common with older adults. Occupational status, cognitive status, or a perceived lack of value of physical activity or of some BCTs are all promising explanations that warrant further investigation.