Aging impacts cancer development with incidence rising exponentially. Age-related genetic and epigenetic changes, along with the aging microenvironment, contribute to cancer development. In older individuals, tumours manifest a heightened mutational burden and distinct genetic changes which differ significantly from tumours observed in younger patients. The aging microenvironment accumulates senescent cells, altered matrix, and a dysregulated immune system. Age-related changes in the immune system fuel tumour development and treatment resistance. Understanding these processes is crucial for optimized treatment of older patients with cancer, as discussed in this review.