Analysis of 14,993 adults tracked over 34 years reveals that those with the highest leisure-time physical activity gained BMI 37% slower than inactive individuals between ages 30-60 (+0.075 vs +0.120 kg/m²/year). The most active participants also showed 17% lower obesity risk compared to the least active. Notably, genetic analysis using cardiorespiratory fitness polygenic scores supported these findings, with similar protective patterns emerging independent of lifestyle factors. The protective effects of physical activity on weight management represent one of the most robust relationships in health science, yet this study provides unprecedented granular detail on how these benefits unfold across the lifespan. The research fills critical gaps by demonstrating that activity's weight-protective effects persist into older age, where BMI typically declines due to muscle loss. However, the observational design cannot fully eliminate confounding factors, and the study population was predominantly European. As a preprint awaiting peer review, these findings require validation, though they align strongly with decades of established research on exercise and weight control.