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Decrease in Incidence of Young-Onset Colorectal Cancer Before Recent Increase. The increasing incidence of colorectal cancer in younger adults (aged <50 years) has been widely reported. Using data from the Surveillance, Epidemiology, and End Results Program, we found young-onset colorectal cancer incidence rates decreased from 1975 through about 1990. Decreases were more prominent in the colon, a contrast with more recent increases in rectal cancer. Incidence rates subsequently increased, differing by time period and 5-year age group. This inflection point is consistent with a birth cohort effect and points to early life exposures-accumulated throughout the life course-that may increase cancer risk. Studying early life exposures among persons born after 1960 may advance our understanding of colorectal cancer in younger adults.