Researchers have identified a key factor in mental aging. They’ve also shown that it might be prevented or reversed by fixing a glitch in the immune system’s front-line soldiers. Certain cells of our immune system become increasingly irascible as we grow older. Instead of stamping out embers, they stoke the flames of chronic inflammation. Biologists have long theorized that reducing this inflammation could slow the aging process and delay the onset of age-associated conditions, such as heart disease, Alzheimer’s disease, cancer, and frailty, and perhaps even forestall the gradual loss of mental acuity that happens to nearly everyone.