11043 Background: Combined axillary lymph node dissection and regional nodal irradiation (ALND+RNI) is the greatest risk factor for breast cancer-related lymphedema (BCRL), a lifelong ailment that negatively impacts quality of life. As disability from BCRL may be particularly profound for adults <45 yo with longer life expectancy, we examined the association of age and other sociodemographics with receipt of ALND+RNI to further our understanding of patients at greatest risk for BCRL. Methods: The National Cancer Database (NCDB) was used to examine age-based differences in receipt of ALND+RNI in patients with stage I-III breast cancer treated from 2018-2020. Cohort characteristics were compared by age (<45 yo, 45-64 yo, and 65 yo). Multivariable regression examined the association between age and ALND+RNI adjusting for relevant factors. Sensitivity analyses explored associations between covariates and combined ALND+RNI by age group. Results: Of 155,010 patients, 5.4% were <45 yo, 47.2% were 45-64 yo, and 47.4% were 65 yo. Clinical and pathological node (cN, pN) positive disease, and receipt of ALND, RNI, and ALND+RNI were more common in patients <45 yo (Table). Age <45 yo was independently associated with ALND+RNI (<45 yo OR 1.40; 45-64 yo OR 1.26 vs 65, p<0.001). Residing in areas with a high proportion of high-school graduates (%HSG), Hispanic ethnicity but not race, and lower income were also associated with ALND+RNI in adjusted analyses (all p<0.05). Medicare (p=0.02 vs private insurance) was inversely associated with ALND+RNI. Among clinical factors, pN+ was the greatest risk factor for receipt of ALND+RNI (all p<0.001 vs pN0). Higher cT, cN1 and cN2 categories, lobular tumors, mastectomy, not receiving endocrine therapy, and having chemotherapy were also associated with ALND+ RNI (all p<0.05). Sensitivity analyses showed that while sociodemographics influenced receipt of ALND+RNI for women 45 yos, only clinical factors were significant for ALND+RNI for <45 yo. Among 45-64 yo, high %HSG (p<0.001) and low household incomes (p<0.01) were associated with ALND+RNI. In the 65 yo cohort, high %HSG (p=0.028), and Asian race (p=0.011) were more likely to receive ALND+RNI while Medicare patients were less likely to have ALND+RNI (p=0.012). Conclusions: Our findings suggests that clinical factors predominate in axillary treatment decisions for younger patients; while sociodemographic factors are more prominent for those 45 yo. Given their higher odds of receiving ALND+RNI, BCRL screening may be especially important in patients <45 yo. Further studies are needed to understand the interplay between age and sociodemographics on receipt of ALND+RNI. [Table: see text]