  In this paper we explore the evolution of a PWN while the pulsar is spinning
down. An MHD approach is used to simulate the evolution of a composite remnant.
Particular attention is given to the adiabatic loss rate and evolution of the
nebular field strength with time. By normalising a two component particle
injection spectrum (which can reproduce the radio and X-ray components) at the
pulsar wind termination shock to the time dependent spindown power, and keeping
track with losses since pulsar/PWN/SNR birth, we show that the average field
strength decreases with time as $t^{-1.3}$, so that the synchrotron flux
decreases, whereas the IC gamma-ray flux increases, until most of the spindown
power has been dumped into the PWN. Eventually adiabatic and IC losses will
also terminate the TeV visibility and then eventually the GeV visibility.
