Background: Metabolic syndrome (MetS) is associated with an increased risk of coronary heart diseases and stroke. Results on the association of MetS with dementia and cognitive decline have been inconsistent. Objective: The aim of this study was to examine the association between MetS and longitudinal changes in cognitive function. Methods: Medline, EMBASE, and Scopus databases were searched from inception to June 2013. Longitudinal cohort studies that reported on the association between MetS and change in cognitive function (over two or more time points), were included. Results: Random-effects models were used to assess the pooled effect sizes of longitudinal changes in cognitive function associated with MetS. Thirteen studies were included. The total sample size was 19,522 subjects. Follow up duration ranged from 1 to 16 years. In the total sample, a small association of MetS with cognitive decline was observed (SDM 0.06, 95%CI:-0.001, 0.12; p = 0.05). When age-stratified, a marginal significant association between MetS and cognitive decline was observed in the younger old group (≤70 years; SDM = 0.09, 95%CI:-0.003, 0.19; p = 0.05) but not in the older group (>70 years; SDM = 0.03, 95%CI:-0.05, 0.11; p = 0.48). The meta-regression showed that duration of follow up was not associated with changes in cognitive estimates (β = 0.005; p = 0.30). Conclusions: Age appears to modify the association between MetS and cognitive decline. These results emphasize the importance of age-stratified risk prediction models of dementia in subjects with chronic metabolic disorders.