Background The prevalence of diabetes is increasing in young adults in Asia, but little is known about metabolic control or the burden of associated complications in this population. We assessed the prevalence of young-onset versus late-onset type 2 diabetes, and associated risk factors and complication burdens, in the Joint Asia Diabetes Evaluation (JADE) cohort. Methods JADE is an ongoing prospective cohort study. We enrolled adults with type 2 diabetes from 245 outpatient clinics in nine Asian countries or regions. We classified patients as having young-onset diabetes if they were diagnosed before the age of 40 years, and as having late-onset diabetes if they were diagnosed at 40 years or older. Data for participants' first JADE assessment was extracted for cross-sectional analysis. We compared clinical characteristics, metabolic risk factors, and the prevalence of complications between participants with young-onset diabetes and late-onset diabetes. Findings Between Nov 1, 2007, and Dec 21, 2012, we enrolled 41 029 patients (15 341 from Hong Kong, 9107 from India, 7712 from Philippines, 5646 from China, 1751 from South Korea, 705 from Vietnam, 385 from Singapore, 275 from Thailand, 107 from Taiwan). 7481 patients (18%) had young-onset diabetes, with age at diagnosis of mean 32.9 years [SD 5.7] versus 53.9 years [9.0] with late-onset diabetes (n=33 548). Those with young-onset diabetes had longer disease duration (median 10 years [IQR 3-18]) than those with late-onset diabetes (5 years [2-11]). Fewer patients with young-onset diabetes achieved HbA(1c) concentrations lower than 7% compared to those with late-onset diabetes (27% vs 42%; p<0.0001) Patients with young-onset diabetes had higher mean concentrations of HbA(1c) (mean 8.32% [SD 2.03] vs 7.69% [1.82]; p<0.0001), LDL cholesterol (2.78 mmol/L [0.96] vs 2.74 [0.93]; p=0.009), and a higher prevalence of retinopathy (1363 [20%] vs 5714 (18%); p=0.011) than those with late-onset diabetes, but were less likely to receive statins (2347 [31%] vs 12 441 [37%]; p< 0.0001) and renin-angiotensin-system inhibitors (1868 [25%] vs 9665 [29%]; p=0.006). Interpretation In clinic-based settings across Asia, one in five adult patients had young-onset diabetes. Compared with patients with late-onset diabetes, metabolic control in those with young-onset diabetes was poor, and fewer received organ-protective drugs. Given the risk conferred by long-term suboptimum metabolic control, our findings suggest an impending epidemic of young-onset diabetic complications.