  We show that for a family of problems described by non-linear diffusion
equations an exact calculation of the two time correlation function gives
C(t,t')=f(t-t')g(t'), t>t', exhibiting normal and anomalous diffusions, as well
as aging effects, depending on the degree of non-linearity. We discuss also the
form in which FDT is violated in this class of systems. Finally we argue that
in this type of models aging may be consequence of the non conservation of the
"total mass".
