Do age-related differences in the incidence of mumps deafness reflect a true difference or a misclassification of mumps deafness?

Mumps deafness is a serious complication of mumps and its risk factors have not been evaluated. Takagi et al. suggested that adolescents and adults have an 8.4 times higher incidence of mumps deafness than children. However, they did not validate the outcome definition of mumps and mumps deafness, and the age-related difference they observed is not the true difference in the incidence of mumps deafness.