AIMS/HYPOTHESIS:Current evidence suggests that type 2 diabetes may have a greater impact on those with earlier diagnosis (longer duration of disease), but data are limited. We examined the effect of age at diagnosis of type 2 diabetes on the risk of all-cause and cause-specific mortality over 15 years. METHODS:The data of 743,709 Australians with type 2 diabetes who were registered on the National Diabetes Services Scheme (NDSS) between 1997 and 2011 were examined. Mortality data were derived by linking the NDSS to the National Death Index. All-cause mortality and mortality due to cardiovascular disease (CVD), cancer and all other causes were identified. Poisson regression was used to model mortality rates by sex, current age, age at diagnosis, diabetes duration and calendar time. RESULTS:The median age at registration on the NDSS was 60.2 years (interquartile range [IQR] 50.9-69.5) and the median follow-up was 7.2 years (IQR 3.4-11.3). The median age at diagnosis was 58.6 years (IQR 49.4-67.9). A total of 115,363 deaths occurred during 7.20 million person-years of follow-up. During the first 1.8 years after diabetes diagnosis, rates of all-cause and cancer mortality declined and CVD mortality was constant. All mortality rates increased exponentially with age. An earlier diagnosis of type 2 diabetes (longer duration of disease) was associated with a higher risk of all-cause mortality, primarily driven by CVD mortality. A 10 year earlier diagnosis (equivalent to 10 years' longer duration of diabetes) was associated with a 1.2-1.3 times increased risk of all-cause mortality and about 1.6 times increased risk of CVD mortality. The effects were similar in men and women. For mortality due to cancer (all cancers and colorectal and lung cancers), we found that earlier diagnosis of type 2 diabetes was associated with lower mortality compared with diagnosis at an older age. CONCLUSIONS/INTERPRETATION:Our findings suggest that younger-onset type 2 diabetes increases mortality risk, and that this is mainly through earlier CVD mortality. Efforts to delay the onset of type 2 diabetes might, therefore, reduce mortality.