Vision-threatening diabetic retinopathy (VTDR) included severe non-proliferative diabetic retinopathy (NPDR), proliferative diabetic retinopathy (PDR) and clinically significant diabetic macular edema (DME). To compare the axial length (AL) and assess its influence on VTDR across different ages.A retrospective cohort study. Medical chart review was performed in 736 consecutive patients with VTDR. The patients were divided into young (≤ 45 years) and elderly group (> 45 years) based on their age at the diagnosis of VTDR. After at least one year of standardized treatments, all eligible patients were followed up. The main outcome measures included the presence of tractional retinal detachment (TRD) involving foveal, final best-corrected visual acuity (BCVA), the development of neovascular glaucoma (NVG), and recurrent vitreous hemorrhage (VH) post-vitrectomy. ALs were compared between two age groups. The impact of AL on clinical outcomes was determined by logistic analyses after controlling for systemic parameters.The study included 144 patients ≤ 45 years and 592 patients > 45 years. Young patients had significantly longer AL than elderly participants (23.9 mm vs 23.0 mm, p < 0.001). Over a median follow-up of 25.9 months, a larger proportion of young patients developed TRD (34.7% vs 16.2%, p < 0.001) and recurrent VH (18.6% vs 10.3%, p = 0.040) than elderly patients. In elderly group, longer AL is an independent protective factor in preventing TRD (odds ratio [OR], 0.5; 95% confidence interval [CI], 0.4-0.7; P < 0.001). However, this beneficial effect was not observed in young patients.Young patients with VTDR exhibited significantly longer AL but more aggressive clinical signs with compromised prognosis. In elderly group, a longer AL independently reduced the risk of TRD, while this protective effect did not exist for young patients.© 2024. The Author(s).