Recent observational longitudinal studies have indicated the association of cognition with lifestyle and lifestyle-related diseases, which can affect timely through the life as protective or risk factors. In particular, inappropriate lifestyle including diet and exercise induces lifestyle-related diseases such as hypertension, diabetes, dyslipidemia, obesity, cigarette smoking which promote cognitive decline and the occurrence of dementia as vascular risk factors. On the other hand, education during early life, occupational exposure during mid-life, and diet with green leafy vegetables and fish oil, and leisure activities including hobbies, social activities, and physical activities during later life could maintain or accelerate the cognitive reserve function. On the basis of modification of lifestyle and management of lifestyle-related diseases, therefore, we should prevent cognitive decline and the occurrence of dementia to achieve healthy aging society.