Ageing results in dysfunction of a number of immune mechanisms, but it particularly undermines primary CD8 T cell responses, which are critical for control of novel intracellular infections and cancers. Our work aims to elucidate how ageing delimits primary CD8 T cell responses through direct effects on naïve CD8 T cells that alter their phenotype and decrease their functionality. As the number of naïve CD8 T cells decreases with ageing, there is an accumulation of a population expressing a semi-differentiated “virtual memory” phenotype. To understand the molecular basis of dysfunctional primary CD8+ T cell responses, we have assessed functional, metabolic and transcriptional differences across the various subsets of naive CD8 T cells from young and aged mice and humans. Understanding characteristics that drive or delimit effective T cell responses allows the optimization or recovery of T cell function via strategies that target these mechanisms.