Abstract Background It has been reported that young patients with diabetes mellitus (DM) had a higher mortality rate after myocardial infarction. However, plaque characteristics among different age groups in patients with acute coronary syndromes (ACS) with or without DM are unknown. Purpose The aim of this study was to investigate the plaque characteristics among different age groups in ACS patients with or without DM in a large population. Methods Patients who presented with ACS and underwent preintervention OCT imaging were included. The culprit plaque was categorized as plaque rupture (PR), plaque erosion (PE) or calcified plaque (CP), and stratified by 5 age groups. Features of plaque vulnerability were also analyzed. Results Among 1394 patients, 482 (34.6%) had DM. DM patients had a higher prevalence of lipid-rich plaque (71.2% vs. 64.8%, p=0.016), macrophage (72.0% vs. 62.6%, p<0.001), and cholesterol crystal (27.6% vs. 19.7%, p<0.001) than non-DM patients. Non-DM patients showed a significant ascending trend with age in PR (p=0.004), lipid-rich plaque (p=0.018), whereas DM patients had a high prevalence of these vulnerable features at early age, which plateaued across the different age groups. Both DM and non-DM groups showed a significant decreasing trend in PE with age (non-DM patients, p<0.001; DM patients, p=0.020). Conclusions DM patients had a high prevalence of lipid-rich plaque and a high incidence of plaque rupture at early age. This pattern persisted across the different age groups.