<p>The U.S. has lower cigarette use but higher marijuana use than global averages. Immigrants to the U.S. are exposed to substance use norms from both their countries of origin and the U.S., with age at immigration potentially determining which norms they adopt. This study examined age at immigration as a determinant of cigarette and marijuana use among U.S. Latinx and Asian young adults. Using data from the 2022–2023 Healthy Minds Study of Asian (<i>N</i> = 8,694) and Latinx (<i>N</i> = 6,128) college students, we used logistic regression models to assess odds of cigarette and marijuana use among students who moved to the U.S. during childhood (before age 12), adolescence (ages 12–17), and young adulthood (ages 18–30), compared to co-ethnic U.S.-born peers. Young adults who moved to the U.S. during young adulthood had lower odds of marijuana use but higher odds of cigarette smoking than U.S.-born peers. Childhood arrivals resembled U.S.-born peers in substance use. Adolescent arrivals had lower marijuana use. For cigarette use, Asian adolescent arrivals had higher use than U.S.-born Asians, whereas Latinx adolescent arrivals showed no difference from U.S.-born Latinx. Developmental timing of immigration shapes substance use among Asian and Latinx young adults potentially through differential exposures to substance use norms and acculturation experiences. Immigration before versus after adolescence appears to be a critical period determining norm adoption. Campus wellness programs should consider age at immigration when assessing foreign-born students’ health needs.</p>