Purpose Most metastatic colorectal cancer (mCRC) patients are elderly. This systematic review identifies and describes observational studies evaluating the influence of age on first-line treatment effectiveness in real-world practice. Methods Medline and EMBASE were searched up to May 2016. The included studies were those that investigated first-line treatment of mCRC and reported age groups and overall survival (OS), progression-free survival (PFS) or overall response rate (ORR) were included. Studies published before 2008 were excluded. Study quality was assessed using the Newcastle–Ottawa Scale. Data were evaluated by age group (< 70 vs. ≥ 70 years; 65–75 vs. ≥ 75 years) and outcome. A pooled survival median was calculated for patients (cutoff = 70 years). Results In total, 11 articles with 11,063 patients were included. Four studies using a cutoff of 70 years of age reported OS and PFS, and two studies reported ORRs. In terms of OS, all studies showed a higher OS for those < 70 years of age than for those ≥ 70 years of age. PFS did not find differences by age. For ORRs, one study favoured the younger group, while the second study did not differ by age. Based on three studies, the pooled medians for  < 70 years of age and ≥ 70 years of age were the same for PFS (10.2) and were 27.0 and 22.9 for OS, respectively. All included studies were of high or acceptable quality. Conclusions The results suggest that age has no effect on PFS. For ORR, the results were inconsistent between studies. Younger patients in general had better OS, which might be partly explained by more aggressive treatment. This treatment seemed not to be guided by performance status or number of metastatic sites. Electronic supplementary material The online version of this article (10.1007/s00432-019-02948-6) contains supplementary material, which is available to authorized users.