<p>Cognitive aging is a multifactorial process characterized by deterioration in attention, memory, learning ability, information processing speed, decision-making, language skills, and executive function. The underlying mechanisms remain unclear. The literature contains data on age-related changes in brain structure, metabolism, and activity, weakening of plasticity, and the influences of genetic and environmental factors. Although considerable evidence of natural changes in the aging brain has been obtained, clear integration of these processes is lacking. Understanding the mechanisms of these processes will enable the development of strategies for the prevention and treatment of cognitive impairment in old age to achieve a full quality of life at any stage. Neurophysiologists distinguish between “healthy aging,” characterized by minor changes in cognitive functions, and “pathological aging,” which is exacerbated by the development of neurodegenerative diseases such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), with sharp decline in mental abilities, leading to dementia. Changes in the brains of patients with AD and PD differ from those observed during normal aging. However, the mechanisms by which aging increases the risk of developing AD and PD remain unclear. To slow or prevent the processes of cognitive decline, it is important to understand the compensatory mechanisms of the brain (cognitive reserve) that increase resilience to cognitive aging. This review briefly describes the main neural and network changes seen in healthy older people and leading to certain difficulties in memory, problem-solving, and decision-making. The main focus is on consideration of the putative causes of cognitive aging. Part of the review concerns the contribution of aging mechanisms to the development of such neurodegenerative diseases as AD and PD. Data on the existing cognitive reserve and approaches to maintaining it are also presented.</p>