Youth gambling has become a significant public health concern, and it appears that individuals are gambling at younger ages than they did in earlier generations. We tested this question by examining birth cohort differences in the age of onset of gambling in a national epidemiologic survey. Data were drawn from the United States National Comorbidity Survey Replication, a nationally representative general population survey of adults born 1904–84. Individuals were divided into four birth cohorts. The cohorts were compared on their lifetime gambling involvement and age of onset of gambling. Significant birth cohort and sex differences were found in the age of gambling initiation, with more recently born cohorts starting to gamble at progressively earlier ages, and men starting to gamble at younger ages than women. The mean age of onset of gambling for individuals born before 1942 was 32.9 years, and for those born between 1973 and 1984 it was 16.9 years. The overall mean ages of onset of gambling were 20.8 for men and 26.4 for women, but more recently born women appear to be ‘catching up’ with their male counterparts. This decreasing age of gambling initiation may help explain the increasing prevalence of disordered gambling in the United States.