  The number of known PWNe has recently increased considerably, and the
majority of them are now middle-age objects. Recent studies have shown a clear
correlation of both X-ray luminosity and size with the PWN age, but fail in
providing a thorough explanation of the observed trends. Here I propose a
different approach to these effects, based on the hypothesis that the observed
trends do not simply reproduce the evolution of a "typical" PWN, but are a
combined effect of PWNe evolving under different ambient conditions, the
leading parameter being the ambient medium density. Using a simple analytic
approach, I show that most middle-age PWNe are more likely observable during
the reverberation phase, and I succeed in reproducing trends consistent with
those observed, provided that the evolution of the X-ray emitting electrons
keeps adiabatic over the whole reverberation phase. As a direct consequence, I
show that the X-ray spectra of older PWNe should be harder: also this is
consistent with observations.
