The increasing incidence of colorectal cancer (CRC) in younger adults (age <50 years) has been widely reported. Using data from the Surveillance, Epidemiology, and End Results Program, we young-onset CRC 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.