This systematic review and meta-analysis examines the association between physical activity and cognitive decline in older adults, focusing on the impact of follow-up length, baseline age, physical activity amount, and study quality. The study included 104 studies with a total of 341,471 participants, assessing both binary outcomes and continuous outcomes of cognition. Key findings include: 1. **Binary Outcomes**: Physical activity was associated with a decreased risk of cognitive impairment or decline (pooled risk ratio, 0.97; 95% CI, 0.97-0.99), but this association was not significant for follow-ups longer than 10 years. Higher-quality physical activity measurements and higher follow-up rates were associated with better cognition. 2. **Continuous Outcomes**: Physical activity was associated with better follow-up global cognition (standardized regression coefficient, 0.03; 95% CI, 0.02-0.03) and change in global cognition (standardized regression coefficient, 0.01; 95% CI, 0.01 to 0.02). There was no significant dose-response association between physical activity and cognition, and no moderators significantly moderated the association. 3. **Specific Cognitive Domains**: Physical activity was associated with improved episodic memory (standardized regression coefficient, 0.03; 95% CI, 0.02-0.04) and verbal fluency (standardized regression coefficient, 0.05; 95% CI, 0.03-0.08). The study suggests that while the association between physical activity and cognitive decline is weak, it is clinically significant from a population health perspective, potentially postponing cognitive decline over decades. However, further high-quality research with longer follow-ups is needed to solidify the evidence.This systematic review and meta-analysis examines the association between physical activity and cognitive decline in older adults, focusing on the impact of follow-up length, baseline age, physical activity amount, and study quality. The study included 104 studies with a total of 341,471 participants, assessing both binary outcomes and continuous outcomes of cognition. Key findings include: 1. **Binary Outcomes**: Physical activity was associated with a decreased risk of cognitive impairment or decline (pooled risk ratio, 0.97; 95% CI, 0.97-0.99), but this association was not significant for follow-ups longer than 10 years. Higher-quality physical activity measurements and higher follow-up rates were associated with better cognition. 2. **Continuous Outcomes**: Physical activity was associated with better follow-up global cognition (standardized regression coefficient, 0.03; 95% CI, 0.02-0.03) and change in global cognition (standardized regression coefficient, 0.01; 95% CI, 0.01 to 0.02). There was no significant dose-response association between physical activity and cognition, and no moderators significantly moderated the association. 3. **Specific Cognitive Domains**: Physical activity was associated with improved episodic memory (standardized regression coefficient, 0.03; 95% CI, 0.02-0.04) and verbal fluency (standardized regression coefficient, 0.05; 95% CI, 0.03-0.08). The study suggests that while the association between physical activity and cognitive decline is weak, it is clinically significant from a population health perspective, potentially postponing cognitive decline over decades. However, further high-quality research with longer follow-ups is needed to solidify the evidence.