Contrast sensitivity under photopic conditions declines with age; however, the cause of this decline remains unknown. To address this issue, we measured detection thresholds for sine wave gratings in noise, under various conditions of spatial-frequency uncertainty, and estimated observers' internal noise and calculation efficiency. Statistical analyses revealed that efficiencies were lower for old (median age at 68 years) than for young (median age at 22 years) observers; no significant differences in internal noise were found. A control experiment ruled out the possibility that reduced retinal illuminance causes the decline in efficiency with age. Our results demonstrate that age-related neural changes play a major role in the decline in contrast sensitivity with age. Possible contributing mechanisms are considered.