  In this paper, we consider the infinite dimensional linear control system
describing population models structured by age, size, and spatial position. The
diffusion coefficient is degenerate at a point of the domain or both extreme
points. Moreover, the control is localized in the space variable as well as
with respect to the age and size. For each control support, we give an estimate
of the time needed to control the system to zero. We establish the null
controllability of the model by using a technique that avoids the explicit use
of parabolic Carleman estimates. Indeed, our argument relies on a method that
combines final-state observability estimates with the use of the characteristic
method.
