The issue about when cognitive aging onset has been a hot point both in aging psychology and gerontology. The answer to this question is critical in determining the optimum time of cognitive intervention. According to three large scale longitudinal studies (Seattle Longitudinal Study, Betula Project, Virginia Cognitive Aging Project), longitudinal studies show that the onset of average decline in cognitive abilities occurs at considerably later ages (e.g. age 50～60) than that suggested by cross-sectional studies (e.g. age 20～30). Moreover, the debate of onset of cognitive decline was discussed from the perspective of quasi-longitudinal，which reveals nearly monotonic declines in cognitive performance from early adulthood. However, the answer for the timing of cognitive decline is elusive now. Future research on the timing of cognitive aging should clarify the methods, and combine the neurobiological study in the cognitive performance.