Population aging is one of the most significant demographic transitions of the twenty-first century, resulting in increased prevalence of age-related cognitive decline and neurodegenerative disorders. Cognitive decline encompasses gradual deterioration in memory, attention, executive function, language, and processing speed that may occur as part of normal aging or as a consequence of pathological conditions such as mild cognitive impairment (MCI), Alzheimer's disease, vascular dementia, and other neurodegenerative disorders. The growing burden of cognitive impairment has profound implications for healthcare systems, caregivers, and society. Advances in neuroscience, neuroimaging, biomarkers, and digital health technologies have improved understanding of cognitive aging and facilitated earlier detection of cognitive changes. This review examines the epidemiology, biological mechanisms, risk factors, assessment methods, prevention strategies, and clinical management of cognitive decline in aging populations. The findings highlight the importance of early intervention, healthy lifestyle promotion, and multidisciplinary care approaches in preserving cognitive health and improving quality of life among older adults.