The immune system undergoes substantial changes throughout life, with ageing broadly impacting immune cell composition, function and regenerative capacity. Emerging evidence suggests that age-associated changes in immune fitness - the ability to respond to and eliminate infection, pathogens and malignancy while maintaining self-tolerance - reshape antitumour immunity and influence the efficacy of immunotherapies. Technological advances in high-dimensional immunoprofiling have begun to reveal the complex interplay between ageing, immune fitness and cancer biology, uncovering new therapeutic vulnerabilities and challenges. In this Review, we discuss recent insights derived from age-resolved immunoprofiling of the human tumour microenvironment, how ageing haematopoiesis affects immune cells that contribute to the microenvironment and impact cancer progression, and what is known from preclinical modelling about the functional consequences of immune ageing on tumour control. We further highlight emerging age-stratified analyses of treatment responses, which are beginning to inform hypotheses about how ageing shapes immunotherapy outcomes. Together, these perspectives provide a framework for integrating age as a critical biological variable, underscore the need to consider age in both preclinical models and clinical trial design, and identify key challenges and priorities for the field moving forward.