Because most patients with cancer are aged and because immunological functions are altered during aging, it is important to account for aging‐associated immunological alterations in the design of new cancer immunotherapies. We thus compared immune populations in young and aged mice and found that B7‐DC+ (PD‐L2/CD273) B cells, a minor population in young mice, were significantly increased in aged mice. Induction of both Th1 and Th17 cells was significantly augmented by B7‐DC+ B cells from aged mice, and this effect was blocked with anti‐B7‐DC antibodies in vitro and in vivo. Moreover, retardation of tumor growth in aged mice was largely B7‐DC dependent. Tumor growth in young mice was significantly inhibited by immunization with B7‐DC+ B cells from aged mice owing to increased induction of tumor antigen‐specific cytotoxic T lymphocytes. These data indicate that B7‐DC+ B cells could play an important role in aging‐associated cancer immunopathology as well as in other aging‐associated diseases and further suggest that B7‐DC+ B cells have potential for future cancer immunotherapy.