Background & Aims: Idiopathic chronic pancreatitis (ICP) is the second most common subtype of CP. In 1994, researchers reported the bimodal age at onset of ICP symptoms: early onset ICP (EO-ICP; median age, 19.2 y) and late-onset ICP (LO-ICP; median age, 56.2 y). Ages of onset and clinical features of ICP differed from those of alcohol-related CP (ACP). However, variants in PRSS1 had not yet been associated with ICP. We reexamined ages of onset of ICP in a large, North American cohort of patients, and investigated the effects of genetic factors and alcohol use in patients with EO-ICP, LO-ICP, and ACP. Methods: We performed a cross-sectional analysis of patients with CP of European ancestry enrolled in the North American Pancreatitis Study 2, a prospective study of 1195 patients with CP from 26 centers in the United States from August 2000 through December 2014. We compared age at onset of symptoms for 130 patients with CP who were lifetime abstainers from alcohol (61 patients with early onset and 69 patients with late onset), 308 light to moderate alcohol drinkers with CP, and 225 patients with ACP and heavy to very heavy alcohol use. DNA from available patients was analyzed for variants associated with CP in SPINK1, CFTR, and CTRC. The Kruskal–Wallis test was used to compare continuous variables across groups and based on genetic variants. Results: Median ages at onset of symptoms were 20 years for patients with EO-ICP and no alcohol use, 58 years for patients with LO-ICP and no alcohol use, 47 years for light to moderate alcohol drinkers with CP, and 44 years for patients with ACP. A higher proportion of patients with EO-ICP had constant pain (65%) than patients with LO-ICP (31%) (P =.04). A higher proportion of patients with ACP had pseudocysts (43%) than patients with EO-ICP (11%) (P =.001). A higher proportion of patients with EO-ICP had pathogenic variants in SPINK1, CFTR, or CTRC (49%) than patients with LO-ICP (23%), light to moderate alcohol drinking with CP (26%), or ACP (23%) (P =.001). Among patients with variants in SPINK1, those with EO-ICP had onset of symptoms at a median age of 12 years, and light to moderate alcohol drinkers with CP had an age at onset of 24 years. Among patients with variants in CFTR, light to moderate alcohol drinkers had an age at onset of symptoms of 41 years, but this variant did not affect age at onset of EO-ICP or ACP. Conclusions: We confirmed previously reported ages at onset of symptoms for EO-ICP and LO-ICP in a North American cohort. We found differences in clinical features among patients with EO-ICP, LO-ICP, and ACP. Almost half of patients with EO-ICP have genetic variants associated with CP, compared with approximately one quarter of patients with LO-CP or ACP. Genetic variants affect ages at onset of symptoms in some groups.