The aim was to evaluate differences in stage, treatment and prognosis in patients aged less than 50 years with colorectal cancer compared with older age groups. This population-based study included all patients diagnosed with colorectal cancer in Sweden, 2010-2015. Disease stage, treatment, 5-year disease-free survival (DFS) and relative survival were analysed in relation to age groups: less than 50, 50-74 and at least 75 years. Of 34 434 patients included, 24·1, 19·7 and 14·0 per cent of patients aged less than 50, 50-74 and at least 75 years respectively were diagnosed with stage IV disease (P < 0·001). Adverse histopathological features were more common in young patients. Among patients aged less than 50 years, adjuvant chemotherapy was given to 18·9, 42·0 and 93·9 per cent of those with stage I, III and III disease respectively, compared with 0·7, 4·4 and 29·6 per cent of those aged 75 years or older (P < 0·001). Stage-adjusted DFS at 5 years for patients under 50 years old was 0·96, 0·90 and 0·77 in stage I, II and III respectively. Corresponding proportions were 0·88, 0·82 and 0·68 among patients aged 50-74 years, and 0·69, 0·62 and 0·49 for those aged 75 years or older. Relative survival was better for young patients only among those with stage III disease. Patients younger than 50 years with colorectal cancer had a poorer stage at diagnosis and received more intensive oncological treatment. DFS was better than that among older patients in early-stage disease. © 2020 BJS Society Ltd Published by John Wiley & Sons Ltd.