Background: Breast cancer is the most commonly diagnosed cancer among women—either active duty, retirees, or dependents—in the U.S. Military Health System (MHS). Surgical treatment is an essential component of treatment for patients without metastatic disease. Differences in insurance coverage and access to care by patient age in the general U.S. population may affect breast cancer diagnosis and surgery. The impact of age on surgical outcomes in breast cancer under conditions of universal healthcare access remains unclear. This study compared postoperative outcomes following breast cancer surgery according to patient age at diagnosis in the U.S. MHS, which provides universal access to care to all its beneficiaries with no or minimal out-of-pocket cost. Methods: We identified women aged 18 and older with stage I–III breast cancer between 2001 and 2014 undergoing partial or total mastectomy without reconstruction in the MilCanEpi database. Multivariable Poisson regression estimated the adjusted risk ratios (ARRs) with 95% confidence intervals (CIs) in association with age at diagnosis (18-39, 40-49, 50-64, and ≥65) for 30-day general and breast complications (surgical site infection, seroma, hematoma, or lymphedema), reoperation, and hospital readmission while controlling for potential confounders. Results: The study included 7,835 women who were 18–39 (9.2%), 40–49 (23.6%), 50–64 (42.7%), and ≥65 (24.5%). Women 18-39 were more likely to present with stage III tumors (20.2%) and high-grade tumors (51.5%) than other age groups. While older women (≥65) had higher unadjusted rates of any complication (RR=1.34; 95% CI=1.12-1.60) or general complication (RR=1.77; 95% CI=1.25-2.50) relative to women diagnosed at age 50-64, these differences were not statistically significant after multivariable adjustment for demographic, tumor, and treatment variables and time to surgery. The overall risk of any breast complication did not differ significantly by age. However, women aged 40–49 had a statistically increased adjusted risk of seroma (ARR=1.50; 95% CI=1.03–2.18) compared to women age 50-64. No significant age-related differences were observed for reoperation or hospital readmission after adjustment. Conclusion: In the universal-access Military Health System, the risk of 30-day postoperative complications and hospital readmissions varied by age among women undergoing surgery without reconstruction for nonmetastatic breast cancer. However, these differences were attenuated after adjustment for clinical and demographic factors, suggesting that age may play a limited role in short-term surgical outcomes for breast cancer. Surgical decision-making should continue to prioritize tumor characteristics, comorbidity burden, and other clinical factors rather than age alone when assessing perioperative risk.