Aged microglia contribute to maladaptive inflammation, but little is known about their progression from homeostasis to dysfunction during aging. Here, we analyze the spatiotemporal kinetics of microglial aging in the hippocampus. Spatially, the dynamics of age-related inflammatory changes in microglia vastly differ across adjoining regions. Using single cell RNA-Sequencing and in vitro approaches, we find that microglia aging proceeds progressively through functional intermediate states that are necessary for inflammatory activation. Overall design: Single cell profiling of Cd11b+ cells in the hippocampus at 6-, 12-, 18-, and 24-months