  The evolution of a pulsar wind nebula (PWN) depends on properties of the
progenitor star, supernova, and surrounding environment. As some of these
quantities are difficult to measure, reproducing the observed dynamical
properties and spectral energy distribution (SED) with an evolutionary model is
often the best approach in estimating their values. G21.5-0.9, powered by the
pulsar J1833-1034, is a well observed PWN for which previous modeling efforts
have struggled to reproduce the observed SED. In this study, we reanalyze
archival infrared (IR; Herschel, Spitzer) and X-ray (Chandra, NuSTAR, Hitomi)
observations. The similar morphology observed between IR line and continuum
images of this source indicates that a significant portion of this emission is
generated by surrounding dust and gas, and not synchrotron radiation from the
PWN. Furthermore, we find the broadband X-ray spectrum of this source is best
described by a series of power laws fit over distinct energy bands. For all
X-ray detectors, we find significant softening and decreasing unabsorbed flux
at higher energy bands. Our model for the evolution of a PWN is able to
reproduce the properties of this source when the supernova ejecta has a low
initial kinetic energy $E_{\mathrm{sn}} \approx 1.2 \times
10^{50}\,\mathrm{ergs}$ and the spectrum of particles injected into the PWN at
the termination shock is softer at low energies. Lastly, our hydrodynamical
modeling of the SNR can reproduce its morphology if there is a significant
density increase of the ambient medium ${\sim} 1.8$ pc north of the explosion
center.
