PURPOSE: The prevalence of diabetes in the United States is increasing. There is also concern that diabetes may be occurring at a greater frequency in youth and in young adults. We describe US population trends in self-reported age at diagnosis of type 2 diabetes mellitus. METHODS: We undertook a secondary analysis of data from the National Health and Nutrition Examination Survey (NHANES) 1999-2000 and NHANES III (1988-1994). Both surveys are stratified, multistage probability samples targeting the civilian, noninstitutionalized US population, which allow calculation of population estimates. We included adults aged 20 years and older. We compared self-reported age at diagnosis of type 2 diabetes between the 2 survey periods. RESULTS: The mean age at diagnosis decreased from 52.0 to 46.0 years (P <.05). Racial and ethnic differences in age at diagnosis found in 1988 to 1994 are no longer found in 1999 to 2000. CONCLUSIONS: The age at diagnosis of type 2 diabetes mellitus has decreased with time. This finding likely represents a combination of changing diagnostic criteria, improved physician recognition of diabetes, and increased public awareness. Younger age at diagnosis may also reflect a true population trend of earlier onset of type 2 diabetes.