  We study a feedback stabilization problem for a first-order hyperbolic
partial differential equation. The problem is inspired by the stabilization of
equilibrium age profiles for an age-structured chemostat, using the dilution
rate as the control. Two distinguishing features of the problem are that (a)
the PDE has a multiplicative (instead of an additive) input and (b) the state
is fed back to the inlet boundary. We provide a sampled-data feedback that
ensures stabilization under arbitrarily sparse sampling and that satisfies
input constraints. Our chemostat feedback does not require measurement of the
age profile, nor does it require exact knowledge of the model.
