Accumulating data suggest that type 2 diabetes mellitus (T2DM) in younger people (aged <40 years), referred to as young-onset T2DM, has a more rapid deterioration of β-cell function than is seen in later-onset T2DM. Furthermore, individuals with young-onset T2DM seem to have a higher risk of complications than those with type 1 diabetes mellitus. As the number of younger adults with T2DM increases, young-onset T2DM is predicted to become a more frequent feature of the broader diabetes mellitus population in both developing and developed nations, particularly in certain ethnicities. However, the magnitude of excess risk of premature death and incident complications remains incompletely understood; likewise, the potential reasons for this excess risk are unclear. Here, we review the evidence pertaining to young-onset T2DM and its current and future burden of disease in terms of incidence and prevalence in both developed and developing nations. In addition, we highlight the associations of young-onset T2DM with premature mortality and morbidity.