Background: Screening for type 2 diabetes mellitus (T2DM) targets people aged 35+ years and those with overweight/obesity. With mounting evidence on young-onset T2DM and T2DM patients with lean phenotypes, it is worth revising the screening criteria to include younger and leaner adults. We quantified the mean age and body mass index (BMI; kg/m2) at T2DM diagnosis in 56 countries. Methods: Descriptive cross-sectional analysis of WHO STEPS surveys. We analysed adults (25–69 years) with new T2DM diagnosis (not necessarily T2DM onset) as per fasting plasma glucose ≥126 mg/dL measured during the survey. For people with new T2DM diagnosis, we summarized the mean age and the proportion of each five-year age group; also, we summarized the mean BMI and the proportion of mutually exclusive BMI categories. Results: There were 8695 new T2DM patients. Overall, the mean age at T2DM diagnosis was 45.1 years in men and 45.0 years in women; and the mean BMI at T2DM diagnosis was 25.2 in men and 26.9 in women. Overall, in men, 10.3% were 25–29 years and 8.5% were 30–34 years old; in women, 8.6% and 12.5% were 25–29 years and 30–34 years old, respectively. 48.5% of men and 37.3% of women were in the normal BMI category. Conclusions: A non-negligible proportion of new T2DM patients were younger than 35 years. Many new T2DM patients were in the normal weight range. Guidelines for T2DM screening may consider revising the age and BMI criteria to incorporate young and lean adults.