Colorectal cancer rates in adults under 50 have doubled since the late 1980s. One in five colorectal cancer diagnoses now occurs in someone under 55, up from one in ten in 1995. A new study highlights the most concrete factor responsible for this trend. Using DNA methylation profiles as molecular records of lifetime exposure, the authors compared cancer patients under 50 with patients over 70 and identified the herbicide picloram as a significant new risk factor. There are converging lines of evidence. The picloram exposure signature was significantly elevated in early-onset tumor tissue and replicated across nine independent patient cohorts. Across 21 years of US county-level data spanning seven states, higher picloram use tracked higher early-onset CRC incidence, and the signal held after adjusting for income, education, and the use of other pesticides. Picloram associated tumors also showed a distinct molecular profile, with APC mutations at 74% versus 90% in low-exposure tumors, suggesting these cancers follow a different biological pathway than classical CRC. Picloram entered commercial use in 1964. Patients now diagnosed in their 70s were already adults before meaningful exposure was possible. Patients now diagnosed in their 30s and 40s were exposed across childhood and adolescence, the developmental windows when epigenetic programming is most plastic. This study delivers the first triangulation of molecular, ecological, and temporal evidence pointing at a specific environmental driver of one of the steepest cancer trends in modern epidemiology.