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This study examined mean differences in, and relationships among, motivational regulations and physical activity in three different age groups (young adults: 18–24, adults: 25–44, and middle-age adults: 45–64 years) using self-determination theory (Deci & Ryan, 1985).Cross-sectional study.Data from 547 participants who completed a self-report questionnaire were analyzed.Based on multivariate analysis of covariance, motivational regulations and physical activity levels differed across age groups. Regression analyses were conducted while controlling for body mass index, gender, education level and ethnicity. Autonomous motivation (consisting of intrinsic motivation and identified regulation) was a positive correlate of physical activity behavior in each age group. Introjected regulation was a significant positive correlate of physical activity behavior, and external regulation was a significant negative correlate of physical activity behavior for young adults. These correlates were not significant in the models predicting physical activity behavior for adults and middle-age adults.Findings highlight the importance of considering age when studying physical activity motivation since the strength of the associations between the motivational regulations and physical activity behavior varied across age groups. Identifying factors that influence intrinsic motivation and identified regulation for physical activity within each age segment is necessary to develop interventions to increase physical activity behavior across the lifespan.