Brown adipose tissue (BAT)-mediated thermogenesis declines with age. This study reveals that bone marrow-derived pro-inflammatory and senescent S100A8+ immune cells invade BAT during aging, compromising axonal networks. Senescent immune cells secrete S100A8, inhibiting adipose RNA-binding motif protein 3 (RBM3) expression, leading to impaired sympathetic innervation and thermogenic function. Human S100A8+ immune cells also induce BAT dysfunction. The S100A8 inhibitor paquinimod rejuvenates BAT and improves metabolic function in aged mice. Targeting senescent immune cells may improve BAT aging and related metabolic disorders.