The evolution of age at maturity under exploitation competition in a patchy environment is modelled using both an analytical approach and computer simulations. Maturity is defined as the switch from allocating resources to growth to allocating them to reproduction, and fitness is measured as lifetime energy allocation to reproduction. Explicit consideration of resources and their exploitation brings about frequency dependence. As a consequence, whenever two or more individuals jointly exploit a patch, the ESS is to mature later and at larger size than at the age and size maximizing the fitness measure. An adaptive response to the presence of competitors thus aggravates the depression of fecundity resulting from competitive resource depletion. In some cases two individuals in a patch should grow larger and mature later than a single individual in a patch of the same size, even though in the first case the individual resource share is halved. The effects of patch size, number of competitors, within-patch mortality and whole-patch destruction rate on the predicted age and size at maturity are discussed.