Age-related diseases are often attributed to immunopathology, which results in self-damage caused by an inappropriate inflammatory response. Immunopathology associated with early-life inflammation also appears to cause faster ageing, although we lack direct experimental evidence for this association. To understand the interactions between ageing, inflammation and immunopathology, we used the mealworm beetle Tenebrio molitor as a study organism. We hypothesized that phenoloxidase (PO), an important immune effector in insect defence, may impose substantial immunopathological costs by causing tissue damage, in turn accelerating aging. In support of this hypothesis, we found that RNAi knockdown of PO transcripts in young adults reduced inflammation-induced autoreactive tissue damage to Malpighian tubules, and increased adult lifespan. Our work thus provides empirical evidence for a causative link between immunopathological costs of early life inflammation and faster ageing. We also reasoned that if natural selection weakens with age, older individuals should display increased immunopathological costs associated with an immune response. Indeed, we found that while old infected individuals cleared infection faster than young individuals, they displayed exacerbated immunopathological costs and higher post-infection mortality. RNAi-mediated knockdown of PO response reduced immunopathology in older beetles and increased their lifespan after infection. This is the first demonstration of a direct role of immunopathological consequences of immune response during ageing in insects. Our work is also the first report that highlights the pervasive role of immunopathology under diverse contexts of aging and immune response.