AbstractBackgroundAdvanced age is associated with decreased likelihood of colorectal cancer treatment. Here, we investigated the extent to which comorbidities are accountable for this lesser treatment.MethodsUsing population‐based datasets, the pattern of care among CRC cases in South Australia during 2004–2013 was investigated. Models were used to investigate associations of age with each treatment type, and differences in these associations were explored by comorbidity and cancer site.ResultsThe presence of comorbidity was associated with a significantly weaker relationship of age with surgery and chemotherapy. The association of age with surgery also varied for colon and rectal primary cancer sites. Individual comorbidity types varied in their associations with each treatment category. For example, dementia was associated with less chemotherapy provision, however, it was not significantly related to the likelihood of surgery.ConclusionThis study indicates that the association of age with surgical treatment differed significantly by the CRC subsite. Comorbidity moderated the negative association of age with chemotherapy, and less so, with extent of surgery. Results were novel in indicating associations of multiple individual comorbidity types with CRC treatment modalities. The data suggest that different individual comorbidity types may have different effects on treatment and should be studied separately.