This study aims to report what motivates individuals to be physically active, to determine whether motivating factors influence physical activity (PA) levels, and whether this differs across the lifespan. This is a cross-sectional study with 498 individuals: 117 adolescents, 306 adults, and 75 older adults. PA was assessed using Baecke’s questionnaire, and motivating factors for practicing sports were investigated using a scale with twelve questions. The factor analysis identified three motivating factors for sports practice: psychosocial, bodily, and well-being. The scale’s overall reliability and internal consistency indicated a Cronbach’s alpha of 0.885. The analysis of covariance (ANCOVA) adjusted for sex indicated the three factors as having a significant effect on PA (p < 0.050); however, only the well-being factor showed a significant interaction with age groups (p = 0.023, ηp2 = 0.030). Subsequently, the effect of the well-being factor on PA scores in each age group was explored through regression analyses. Only older adults showed a significant association in the unadjusted [OR = 0.378, p = 0.001, R2 = 0.243] and the sex-adjusted analysis [OR = 0.377, p = 0.001, R2 = 0.288]. These results help us to better understand the underlying motivational reasons in different age groups for engaging in sports.