Objectives: First-line chemotherapy for metastatic colorectal cancer (mCRC) is effective and feasible in selected older patients. We investigated age-dependent differences in treatment and outcomes in patients with mCRC in clinical practice. Material and methods: A retrospective study of 654 patients with mCRC referred to first-line chemotherapy in 2008–2014. Patients were divided into two age groups: 50–69 and ≥70 (older patients). Binary outcomes were analyzed by logistic regression. Progression-free survival (PFS) and overall survival (OS) were analyzed by Cox proportional hazards regression, CRC-specific and other-cause mortality with Fine and Gray proportional hazard model for the sub-distribution of a competing risk. Results: After adjusting for performance status (PS) and comorbidity, older patients were more likely to receive monotherapy (adjusted odds ratio (aOR) = 9.00, 95% confidence interval (CI) 4.52–17.91), lower doses, and no additional targeted therapy (aOR = 1.89, 95% CI 1.28–2.78) than younger patients. Yet, older patients experienced more toxicity and hospitalizations (aOR = 1.53, 95% CI 1.08–2.17). Among those treated, older patients had shorter PFS (hazard ratio (HR) = 1.32, 95% CI 1.11–1.57), but after adjusting for PS and comorbidity, PFS was similar. No significant difference was found in CRC mortality (HR = 1.15, 95% CI 0.95–1.40) between age groups. Poor PS was associated with shorter OS and PFS and higher CRC mortality. Conclusions: In the DISCO study, older patients with mCRC received less aggressive first-line chemotherapy. Yet, they experienced more toxicity. Younger and older patients had similar CRC mortality. Shorter PFS and higher CRC mortality were observed in patients with poor PS.