INTRODUCTION: The number of young breast cancer (BC) patients is increasing in both high- and low-income countries. It is known that this population is at risk for more aggressive tumor phenotypes, larger tumor size at diagnosis and poorer prognosis. It is the aim of this population-based analysis to identify trends of therapy, tumor biology and prognosis during a period of 11 years in young patients under the age of 40. METHODS: In this analysis, data of young BC patients (< 40 years) from two breast centers were collected and analysed. The focus was a summary of data regarding tumor phenotypes, treatment, and survival in young BC patients. RESULTS: Out of 11,954 patients with invasive BC who were eligible to the analysis, 781 (6.5%) were younger than 40 years at diagnosis and met the inclusion criteria. The predominant biological subtypes were Luminal B-like (HER2-) and Luminal-A-like, 62.3% were diagnosed with pN0. Noticeably low rates for endocrine therapy and higher rates for chemotherapy could be observed. 10-year overall survival was 87% for the whole cohort. Luminal-B-like (HER2-) and Triple negative tumors had worse outcomes as opposed to the other subtypes. CONCLUSION: As a conclusion, this 11-year analysis provides valuable insights into the clinical characteristics and treatment outcomes of young breast cancer patients under 40 years of age. The analysis highlights clear outcome differences according to the tumor subtype. These findings underscore the need for personalized treatment approaches and continued follow-up to optimize outcomes for young BC patients.