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Several biological processes influence aging, and three main theories explain its mechanisms.

The cellular clock theory proposes that human cells have a limited capacity to divide during their lifespan. As cells age, their ability to divide diminishes, and the total number of cell divisions is closely linked to an individual's lifespan.

Secondly, the free-radical theory of aging proposes that aging results from the production of unstable oxygen molecules, known as free radicals, within cells. 

These molecules cause damage to DNA and other cellular structures, increasing the risk of a range of disorders, including cancer and arthritis.

The hormonal stress theory suggests that aging in the hormonal system decreases stress resistance and increases disease risk.

As individuals age, stress hormones remain in the bloodstream longer, contributing to higher risks of cardiovascular disease, cancer, and diabetes.

Additionally, with aging, specific brain regions like the prefrontal cortex experience declines in cognitive functions like memory.

Conversely, research suggests that older adults' brains can adapt by reorganizing neural pathways to compensate for these losses.