Cancer is more common in older people because—

Cancer is primarily a disease of aging because it results from the gradual accumulation of somatic mutations in the DNA of cells over a lifetime. The multistage model of carcinogenesis suggests that multiple genetic 'hits' or mutations in oncogenes and tumor suppressor genes are required for a normal cell to transform into a malignant one. While the immune system's efficiency does decline with age (immunosenescence), which can reduce the surveillance of cancerous cells [1], the fundamental driver is the time-dependent exposure to mutagens and errors in DNA replication. Older individuals have had more time to accumulate these critical genetic alterations. Additionally, while hormonal changes occur during aging , they are generally specific to certain cancer types rather than the universal cause for increased cancer incidence across all tissues. Therefore, the accumulation of mutations remains the most comprehensive biological explanation for the age-related spike in cancer.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 19: Ozone Depletion > Effects of human and animal health > p. 271