Age is the greatest risk factor for the development of cancer. For etiologic purposes, newly diagnosed cases of cancer among a defined population during a specified time (incidence) is the usual way of depicting cancer as it relates to age. Exposure to carcinogens in utero or perinatally can produce cancers soon after birth or years later. Cancers in older children have been related to growth factors and/or a single exposure to high doses of radiation. Hodgkin's disease occurring among young adults is different histologically, clinically, and prognostically than Hodgkin's disease among older adults. For the disease among young adults, the hypothesis is that clinical disease reflects the rare consequences of a prevalent infection of low pathogenicity; age of infection is determined by socioeconomic status. In older adults, it more closely resembles the lymphomas. This suggests dynamic trends associated with changing social environments related to etiologic factors. Among adults, the steady increase in colon cancer among both genders represents constant exposure to a carcinogen(s) starting in early life and persisting throughout older ages. Breast cancer is divided into pre- and postmenopausal phases on the basis of its age distribution. International differences in postmenopausal breast cancer suggest environmental factors in postmenopausal women and genetic and hormonal factors in premenopausal women. The age distribution of lung cancer increases linearly with the amount of cigarettes smoked and there is no indication of a threshold below which cigarette smoke is safe. The downturn among the oldest age groups results from competing causes of death or reflects a cohort effect of different exposure over time. Further, the pattern of lung cancer suggests exposure to a carcinogenic agent including substances that act principally as promoters.