Advancing age substantially increases cancer risk, primarily due to progressive biological alterations over time. With the global population aging rapidly, the incidence of cancer is also rising. In cancer immunotherapy, patient age is emerging as a critical determinant for both identifying and predicting responses to immune checkpoint inhibitors. Aging is accompanied by significant changes in the immune system, notably a decline in T-cell function and a reduction in tumor-infiltrating lymphocytes, which collectively reshape the tumor microenvironment and weaken antitumor immunity. Immune senescence compromises the ability to recruit and maintain functional TIL activity, thereby limiting the benefits of immune checkpoint inhibitors therapy. Furthermore, senescent tumor cells influence their surroundings by releasing a broad spectrum of pro-inflammatory cytokines and chemokines, a phenomenon termed the senescence-associated secretory phenotype, while simultaneously upregulating immune inhibitory markers such as PD-L1. In addition, age-related immune dysregulation exacerbates cellular exhaustion, leading to abnormal expression of key biomarkers that govern immune checkpoint inhibitors efficacy and ultimately attenuating antitumor immune responses. This perspective discusses the mechanisms through which aging alters systemic immunity and the tumor microenvironment, thereby reducing immunotherapy effectiveness. By integrating current mechanistic insights into the interplay between aging and cancer immunobiology, we highlight potential aging-related biomarkers that may improve therapeutic strategies in geriatric oncology. A deeper understanding of these interactions is essential for developing personalized immunotherapeutic approaches tailored to the unique needs of elderly cancer patients.