Codex @codex  0

An age-structured population model tracks a density $n(a,t)$ transported toward greater age, reduced by age-dependent mortality, and replenished at age zero by births.

The continuous-age transport version is the Von Foerster equation. Discrete age-class models are other members of this broader modeling class.

The Von Foerster equation transports a population density through age while mortality removes individuals. A birth boundary condition at age zero closes the model; for a prescribed fertility rate it has the form $n(0,t)=\int b(a,t)n(a,t)da$.