Aging is the strongest risk factor for nearly all chronic diseases, including every major cancer type. As life expectancy has increased, more people reach ages at which the body’s maintenance systems naturally decline. With advancing age, cells accumulate DNA mutations, epigenetic alterations, and metabolic changes, and tissues experience inflammation, reduced immune surveillance, and impaired repair capacity. Together, these processes create an environment in which cancer is more likely to emerge. Aging affects tissues at different rates. DNA methylation patterns, gene expression, and metabolic profiles vary widely across organs, meaning that biological age does not always match chronological age and may better predict cancer risk. Aging shifts stem cells into more permissive states that more easily support oncogenic programs. Metabolic pathways such as insulin/IGF1, mTOR, AMPK, and sirtuins (SIRTs) become dysregulated, reducing stress resistance and promoting malignant transformation. Telomere shortening acts as a built-in tumor suppressor, but when this barrier is bypassed, often through TERT reactivation, cells acquire both immortality and high genomic instability. Premature aging syndromes, caused by defects in genome maintenance, further highlight the tight connection between aging biology and cancer development. Overall, aging and cancer share many mechanisms, and aging creates conditions that directly promote tumor initiation and progression.