Background The incidence of early-onset colorectal cancer (EOCRC) is increasing. International guidelines state that treatment should not differ from that of older patients. Several studies have shown that patients under 50 years are receiving more aggressive treatment, without any survival benefit. We aim to determine if treatment for stage 2 and 3 EOCRC differs from that of late onset colorectal cancer (LOCRC) patients, and determine if there is any association with outcomes. Methods This was a retrospective, population-based, cohort study of the treatment patterns of patients diagnosed with colorectal cancer in Canterbury, New Zealand from 2010-2021 age <50 years, compared to those aged 60-74 years. Results 3263 patients were diagnosed with CRC between 2010-2021. Following exclusions we identified 130 EOCRC and 668 LOCRC patients. Stage 2 EOCRC patients are more likely to be offered adjuvant chemotherapy (p=<0.001). Furthermore, EOCRC patients with either stage 2 or 3 disease, are more likely to receive multi-agent therapy (p=<0.01), without any associated increase in survival. Conclusion EOCRC patients are given more adjuvant chemotherapy, without a corresponding improvement in outcomes highlighting a potential for increased treatment related harms, particularly in stage 2 CRC. Clinicians should be mindful of these biases when treating young cancer patients, and need to carefully consider treatment related harms.