Introduction and Objective: Individuals with type 1 diabetes (T1D) have a greater risk of complications and reduced life expectancy compared with those without diabetes. It is hypothesized that age at T1D diagnosis impacts clinical complications and mortality. Methods: A nationwide retrospective cohort study was conducted. Individuals ≥18 years of age with T1D with ≥1 entry in the Swedish National Diabetes Register (NDR) from 1998 through 2019 and matched controls were included. T1D status was based on insulin use and diagnosis at ≤30 years of age. Individuals with T1D with coexisting congenital, metabolic, or neurological disorders were excluded. Relationships between age at T1D diagnosis and fatal and nonfatal outcomes (eg, cardiovascular (CV) mortality, acute myocardial infarction (MI), peripheral arterial disease (PAD), end-stage renal disease) were analyzed using Cox proportional hazards models. Results: Individuals with T1D (n=34,155) had a mean age of 30.7 years and median follow-up of 12.9 years. Younger age at T1D diagnosis was associated with a higher risk of mortality and nonfatal outcomes. T1D diagnosis at < 10 years of age was associated with 16.3 life-years lost. The hazard ratio (HR) per 1-year older age at T1D diagnosis was 0.97 (95% CI: 0.97, 0.98) for CV mortality, indicating a 2.6% lower risk for every 1-year increase in age at diagnosis. The HRs per 1-year older age at T1D diagnosis were 0.99 (95% CI: 0.98, 0.99) for non-CV mortality, 0.96 for end-stage kidney disease (95% CI: 0.96, 0.97), 0.97 (95% CI: 0.96, 0.98) for PAD, 0.97 (95% CI: 0.96, 0.98) for acute MI, 0.97 (95% CI: 0.97, 0.98) for heart failure, and 0.97 (95% CI: 0.97, 0.98) for coronary heart disease. T1D diagnosis before age 10 was associated with the greatest risk of CV mortality (HR 5.7; 95% CI: 4.3, 7.6). Conclusion: Early onset of T1D carries increased risk of serious complications and death. These findings support the premise that delaying T1D onset in young individuals may reduce the risk of future complications. Disclosure A. Rawshani: Consultant; Sanofi. B. Eliasson: Other Relationship; Sanofi, Eli Lilly and Company, Novo Nordisk. Speaker's Bureau; Boehringer-Ingelheim. Other Relationship; Amgen Inc. Advisory Panel; Abbott, Bayer Pharmaceuticals, Inc. D.K. McGuire: Consultant; Novo Nordisk. Advisory Panel; Novo Nordisk. Consultant; Lilly USA LLC. Advisory Panel; Lilly USA LLC. Consultant; Boehringer-Ingelheim. Advisory Panel; Boehringer-Ingelheim, AstraZeneca, ESPERION Therapeutics, Inc., NewAmsterdam Pharma, Pfizer Inc. Consultant; Applied Therapeutics, Lexicon Pharmaceuticals, Inc, Bayer Pharmaceuticals, Inc, Amgen Inc, Kailera, Idorsia, Alveus, Metsera. N. Sattar: Other Relationship; Abbott, AbbVie Inc, Amgen Inc, AstraZeneca, Boehringer-Ingelheim, Eli Lilly and Company. Advisory Panel; Hanmi Pharm. Co., Ltd. Consultant; Carmot Therapeutics, Inc, Menarini. Other Relationship; Novartis AG, Novo Nordisk, Pfizer Inc, Roche Diagnostics. Consultant; GlaxoSmithKline plc, Metsera. Other Relationship; American Diabetes Association, Medscape. H.C. Gerstein: Advisory Panel; Abbott, Bayer Pharmaceuticals, Inc, Eli Lilly and Company, Novo Nordisk. Consultant; Pfizer Inc, Sanofi, Hanmi Pharm. Co., Ltd. Research Support; Eli Lilly and Company, Novo Nordisk, Hanmi Pharm. Co., Ltd. Other Relationship; Eli Lilly and Company, Novo Nordisk, Sanofi, Boehringer-Ingelheim, Abbott, Jiangsu Hansen, Zuellig Pharma, AstraZeneca. J. Isufi: None. N. Cui: None. O. Guenther: Employee; Sanofi. J.H. Zaccai: Employee; Sanofi. A. Mahieu: Employee; Sanofi. A. Rawshani: Consultant; Sanofi. Funding This study was funded by Sanofi. Acknowledgments: Medical writing support was provided by Lauren Poppi, PhD, of IMPRINT Science, New York, NY, and was funded by Sanofi.