Aging leads to neurodegenerative diseases, such as cognitive decline, which are induced by persistent chronic low-grade inflammation in the brain driven by microglial activation. However, whether and how brain-derived exosomes from aged mice (A-exo) induce a pro-inflammatory state and cellular senescence in microglia within the aging brain is poorly understood. Here, we report that brain-derived exosomes from aged mice (A-exo) cause cognitive decline in normal young mice, inducing microglial overactivation, lipid droplet accumulation, and senescence-associated secretory phenotype (SASP) secretion. This abnormal microglial activity arises from the elevated expression of PTGDS in A-exo due to mouse aging, resulting in increased central and peripheral D-prostanoid receptor 1 (DP1) ligand PGD2 levels, which subsequently leads to sustained DP1 signaling activation. Consequently, this process promotes myeloid cell infiltration, cellular senescence, and cognitive decline by generating a senescent, pro-inflammatory microglial phenotype. Blocking the DP1 receptor ameliorates A-exo-mediated microglial overactivation, myeloid cell infiltration, and cellular senescence. Strikingly, DP1 receptor blockade improves cellular senescence, neuroinflammation, and cognitive decline in aged mice. Our findings reveal a systemic mechanism underlying the sustained activation of microglia following brain aging, paving the way for improving chronic neuroinflammation, cellular senescence, and cognitive decline associated with aging.