Multiple regression analyses of 2,576 adults in the LIFE-Adult Study revealed no positive associations between physical activity and cognitive performance on Trail Making Tests, hippocampal volume, or brain aging markers. Even objective accelerometry data from 227 participants failed to demonstrate brain benefits. Longitudinal analysis suggested reverse causation—higher brain age at baseline predicted lower physical capacity later, not the reverse. The study also uncovered systematic age-related bias where older adults overestimate their activity levels. These null findings challenge widely accepted assumptions about exercise's cognitive benefits, though several factors warrant caution. The cross-sectional design limits causal inference, and self-reported activity is notoriously unreliable. The cognitive measures were narrow, potentially missing benefits in other domains like memory formation or executive flexibility. Population-based cohorts often include inactive participants across all groups, potentially masking dose-response relationships that emerge only at higher activity thresholds. As a preprint awaiting peer review, these results require independent validation. However, they underscore the complexity of exercise-brain relationships and suggest that intervention timing, intensity thresholds, and measurement precision may be more critical than previously recognized.