Sporadic Alzheimer’s disease, the most common neurodegenerative disorder of aging, is characterized by cerebral plaques and neurofibrillary tangles. Experimental rodents develop plaques but neither tangles nor substantial neurodegeneration under conditions that guarantee Alzheimer’s in humans, suggesting rodents lack critical co-initiation factors. Accumulation of antigen-reactive memory CD8 T cells increases with aging, and was recently revealed as a hallmark of human Alzheimer’s. The impact of this process on disease initiation, however, has not been established because age-related T cell changes are muted in rodents. We developed a mouse model of human-like CD8 T cell aging that promotes antigen-reactive memory CD8 T cell accumulation. Here we show that these “hiT” mice develop all major hallmarks of Alzheimer’s with aging, including tangle-like inclusions and substantial neurodegeneration. Antigen-reactive CD8 T cells analogous to those in hiT mice increased in Alzheimer’s brain, but decreased earlier in blood, where their loss effectively distinguished the Alzheimer’s continuum from aging controls. Our findings establish a clinically relevant mouse model for sporadic Alzheimer’s and show that age-related immune dysfunction critically contributes to its initiation. They also identify useful immune-based targets to track and potentially treat human Alzheimer’s, while validating a model system to examine age-related disease immuno-biology more generally.