Standard treatment of patients with breast cancer decreases with age and older persons are mostly excluded from clinical trials. We hypothesized that non-adherence to treatment guidelines occurs for women aged ≥70 years and changes overall survival (OAS) and disease-free survival (DFS). We enrolled 1922 women aged ≥50 years with histologically confirmed invasive breast cancer treated at the University of Ulm from 1992 to 2005. Adherence to guidelines and effects on OAS and DFS for women aged ≥70 years was compared with that for younger women (50–69 years). Women >70 years less often received recommended breast-conserving therapy (70–79 years: 74%–83%; >79 years: 54%) than women aged ≤69 years (93%). Non-adherence to the guidelines on radiotherapy (<70 years: 9%; 70–79 years: 14%–27%; >79 years: 60%) and chemotherapy (<70 years: 33%; 70–79 years: 54%–77%; > 79 years: 98%) increased with age. Omission of radiotherapy significantly decreased OAS ≤69 years: hazard ratio (HR) = 3.29; P <0.0001; ≥70 years: HR = 1.89; P = 0.0005 and DFS (≤69 years: HR = 3.45; P <0.0001; ≥70 years: HR = 2.14; P <0.0001). OAS and DFS did not differ significantly for adherence to surgery, chemotherapy, or endocrine therapy. Our study confirms that substandard treatment increases considerably with age. Omission of radiotherapy had the greatest impact on OAS and DFS in the elderly population.