A large class of diseases is dependent on juvenile hosts for transmission because younger hosts are typically more susceptible to disease. Studies have investigated the epidemiological consequences of juvenile susceptibility, but why species retain such high susceptibility in the juvenile stage remains a puzzle. Life-history theory predicts that hosts should evolve to be more resistant as juveniles than as adults because early infection is costlier. Studies of anther-smut on wild carnations show that disease persistence is strongly dependent on the presence of a highly susceptible juvenile class. While there is evidence of genetic variation in juvenile resistance, the majority of plant families are highly susceptible at this stage, so juvenile resistance may be less beneficial than assumed. To understand how the costs and benefits of resistance and life-history traits affect the evolution of age-specific resistance, we developed a general analytical model of age-specific resistance, which shows that if there is genetic variation for the onset of resistance, selection and numerical feedbacks often drive the evolution of adult resistance but maintain juvenile susceptibility. The implications of these results are discussed.