Immunosenescence, the age-related decline in immune function, profoundly impacts cancer progression and therapeutic outcomes by fostering a tumor-promoting microenvironment and impairing immune surveillance. This review delineates eleven molecular hallmarks of immunosenescence, including genomic instability, telomere attrition, epigenetic dysregulation, mitochondrial dysfunction, and chronic inflammation, which collectively drive immune cell dysfunction and systemic immunosuppression. Aging reshapes the tumor microenvironment (TME) through recruitment of immunosuppressive cells, senescence-associated secretory phenotypes (SASP), and metabolic reprogramming, contributing to therapy resistance and poor prognosis in elderly patients. While immunotherapies such as immune checkpoint inhibitors (ICIs) and chimeric antigen receptor T-cell immunotherapy (CAR-T) cells show promise, their efficacy in aging populations is limited by T cell exhaustion, myeloid bias, and altered intercellular communication. Emerging strategies-including senolytics, epigenetic modulators (e.g., histone deacetylase (HDAC) inhibitor), and metabolic interventions (e.g., spermidine, nicotinamide mononucleotide (NMN))-highlight potential avenues to rejuvenate aged immunity. Single-cell multi-omics (single cell RNA-seq, single cell ATAC-seq) further unravel immune cell heterogeneity, revealing tissue-specific chromatin accessibility dynamics and novel targets like interleukin-34 (IL-34) for microglia-mediated neuroinflammation. However, challenges persist in translating preclinical findings to clinical practice, necessitating age-tailored trials and biomarker-driven approaches. By integrating mechanistic insights with translational innovations, this review underscores the urgency of addressing immunosenescence to optimize cancer immunotherapy for aging populations, ultimately bridging the gap between aging biology and precision oncology.