The promise that regular exercise slows biological aging through epigenetic mechanisms faces a critical evidence gap that could reshape how we interpret fitness studies. Despite widespread enthusiasm for DNA methylation clocks as aging biomarkers, the causal relationship between physical activity and improved epigenetic age remains scientifically unproven. This commentary in The Lancet Healthy Longevity challenges researchers to move beyond observational associations toward rigorous causal evidence. The analysis highlights how first-generation epigenetic clocks like Horvath's measure chronological age prediction, while newer versions like DNAm GrimAge incorporate mortality and clinical biomarkers for health assessment. However, these tools may reflect correlation rather than causation when linked to exercise interventions. The fundamental issue lies in distinguishing whether physical activity directly modifies DNA methylation patterns that drive aging, or whether healthier individuals who exercise simply possess different baseline epigenetic profiles. This distinction carries profound implications for longevity science and public health messaging. Current observational studies show exercisers have 'younger' epigenetic ages, but this could reflect selection bias, genetic predisposition, or confounding lifestyle factors rather than exercise-induced biological changes. The commentary underscores a broader challenge in aging research: translating promising biomarker associations into actionable causal mechanisms. Without randomized controlled trials specifically designed to test epigenetic outcomes, the field risks overstating exercise benefits based on correlational evidence. This represents a crucial methodological awakening for researchers studying exercise and longevity, demanding more sophisticated study designs to establish whether physical activity truly reprograms our biological clocks or merely correlates with favorable aging profiles.