Epidemiological data indicate high comorbidity between diseases linked to serum lipid levels, such as atherosclerosis, and cognitive decline. However, it is difficult to reduce the effects of poor general lifestyle to specific mechanisms that can produce these deficits. Here we used an animal model to determine the relation between serum lipids and cognitive decline in a population whose diet was stable across the lifespan. Serum lipids and cognitive performance were assessed in 61 African Green Monkeys, ranging in age from 3–24 years and maintained on a standard diet (high fiber, low fat, high protein) over the lifespan; female subjects were not pregnant at sampling. Learning and response inhibition were measured using a visual discrimination/reversal task. For those animals in the highest quintile for age (17–24 years), serum low density lipids were negatively associated and serum high density lipids were positively correlated with cognitive performance on all indices tested (p <0.05). These relationships were not evident for younger subjects. Thus, the relation between serum lipids and cognitive performance is not independent of age and is present in individuals who maintain a healthy diet across the lifespan. Research supported by P40 RR019963, RL1 MH 083270, UL1‐DE019580.