The McKendrick–Von Foerster equation (1926/1959) tracks population density n(a,t) by age a and time t. Extended to epidemics: susceptibles S(a,t), infected I(a,t), recovered R(a,t) with age-dependent rates. The force of infection λ(a,t) = ∫ β(a,a')I(a',t)da' couples age groups through a mixing matrix β. Age structure profoundly affects herd immunity threshold and optimal vaccination strategies.