  The distribution of a population throughout the physiological age of the
individuals is very relevant information in population studies. It has been
modeled by the Langevin and the Fokker- Planck equations. A major problem with
these equations is that they allow the physiological age to move back in time.
This paper proposes an Infinitesimally ratcheted random walk as a way to solve
that problem. Two mathematical representations are proposed. One of them uses a
non-local scalar field. The other one is local, but involves a multi-component
field of speed states. These two formulations are compared to each other and to
the Fokker-Planck equation. The relevant properties are discussed. The dynamics
of the mean and variance of the population age resulting from the two proposed
formulations are obtained.
