Breast cancer in younger patients appears to be more aggressive than disease occurring in older patients. Even though large population-based studies suggest poorer survival of patients younger than 35 years, data demonstrating the relationship of age and prognosis within premenopausal cohorts are much more scarce and conflicting. In this retrospective analysis of 885 premenopausal patients, the relationship between age, typical prognostic factors, treatment, and patient outcome was investigated. Eight hundred four patients (90.8%) > 35 years and 81 patients (9.2%) = 35 years who had been treated for stage I/II breast cancer were evaluated. Median follow-up time was 71 months. The prevalence of adverse prognostic features such as tumor size, tumor type, tumor grading, pathologic lymph node status, and hormone receptor status were evenly distributed between the two age groups. Age = 35 years proved to be a powerful independent prognostic factor in multivariate analyses of recurrence-free (P < 0.0001; relative risk [RR] = 2.5) and overall survival (P < 0.0039, RR = 2.2). Thus, in the face of evenly distributed risk factors in this strictly premenopausal, homogeneous population, young age was seen as the second most powerful risk factor after lymph node status. According to these findings, patients diagnosed with breast cancer at = 35 years of age have a worse prognosis compared to premenopausal women above this age. Future studies should focus on unveiling the young age surrogate in order to improve treatment and prognosis.