Objectives: We investigated whether social-cognitive variables predicted physical activity behavior and whether Internet use interrelated with the mediator, across age groups. Methods: Participants (N = 466) were recruited for an online survey and path analyses were run to generate a proposed model examining late adolescents (Group 1, aged 17-20 years), young adults (Group 2, aged 21-40 years), and middle-aged adults (Group 3, aged 41-60 years). Results: Positive associations between physical activity variables were as expected: the relationships between self-efficacy and intention, self-efficacy and behavior, planning and behavior were statistically significant in Groups 1 and 2. The relationships between intention and planning were statistically significant in Groups 2 and 3. All groups showed statistically significant relationships between self-efficacy and planning. Positive associations between Internet use and physical activity emerged only in Group 1. Internet use was negatively correlated with physical activity planning only in Group 3. Conclusions: Late adolescents should be supported to enhance physical activity planning, and middle-aged adults may benefit from support in reducing the possible conflicts between physical activity planning and time spent online. Furthermore, increasing self-efficacy for all groups is key to adopting and maintaining physical activity.