Age-associated insulin resistance (IR) and obesity-associated IR are two physiologically distinct forms of adult-onset diabetes. While macrophage-driven inflammation is a core driver of obesity-associated IR1-6, the underlying mechanisms of the obesity-independent yet highly prevalent age-associated IR7 are largely unexplored. Here we show, using comparative adipo-immune profiling in mice, that fat-resident regulatory T cells, termed fT(reg) cells, accumulate in adipose tissue as a function of age, but not obesity. Supporting the existence of two distinct mechanisms underlying IR, mice deficient in fT(reg) cells are protected against age-associated IR, yet remain susceptible to obesity-associated IR and metabolic disease. By contrast, selective depletion of fT(reg) cells via anti-ST2 antibody treatment increases adipose tissue insulin sensitivity. These findings establish that distinct immune cell populations within adipose tissue underlie ageing- and obesity-associated IR, and implicate fT(reg) cells as adipo-immune drivers and potential therapeutic targets in the treatment of age-associated IR.