Some once common cancers have become rare. For example, cancer of the stomach was four times more prevalent in the United States in 1930 than it is today, probably because people today consume much less smoked, pickled, and spoiled food. On the other hand, lung cancer occurrence in the United States increased from 5 people per 100,000 in 1930 to 114 people per 100,000 in 1990, and the rate of lung cancer in women has skyrocketed. These changes are almost certainly the result of increased cigarette smoking. Cigarette smoking has also led to an increase in cancers of the mouth.
Age is an important factor in the development of cancer.
Some cancers, such as Wilms' tumor, acute lymphocytic leukemia, and Burkitt's lymphoma, occur almost exclusively in young people. Why these cancers occur in the young is not well understood, but genetic predisposition is one factor. However, most cancers are more common in older people. Many cancers, including those of the prostate, stomach, and colon, are most likely to occur after age 60. Over 60 percent of the cancers diagnosed in the United States are in people over 65 years of age.