Across a wide range of species, juveniles are more susceptible to pathogenic infection than their adult conspecifics, often driving epidemic spread. Theory has shown that high innate juvenile susceptibility can be favoured by natural selection given a trade-off with other host fitness traits such as reproduction, facilitating pathogen persistence. However, most research on age-specific infection has focused solely on host evolutionary dynamics, and the effect of coevolution on juvenile infection rates has not been explored. Here, we developed a coevolutionary adaptive dynamics model, loosely based on a model plant disease system, to investigate age-specific infectivity and contrast the outcome of coevolution with host evolution and pathogen evolution alone. We found strong differences in the evolutionary dynamics of hosts and pathogens in the absence of coevolution. While host evolution often favoured genotypes with intermediate levels of juvenile and adult resistance, pathogen evolution selected for extreme specialists. When hosts and pathogens were allowed to coevolve, the dynamics more closely resembled the pathogen dynamics; pathogens rapidly evolved to specialize on infecting a specific age class, and this drove reciprocal specialization in host resistance. Furthermore, we found that the form of specialist favoured by selection can depend on the starting host and pathogen genotypes, but only with coevolution. Our results underscore the importance of understanding both the host and pathogen contributions to age-specific infection rates.