There has been a sharp rise in colorectal cancers in adults as young as in their 20s and 30s in the U.S., found a new study from the American Cancer Society . The vast majority of colorectal cancers are still found in older people, with nearly 90 percent of all cases diagnosed in people over 50, colorectal cancer rates, which had dropped steadily for people born between 1890 and 1950, have been increasing for every generation born since 1950, said the study published on Tuesday.