  The bright Type II-plateau supernova (SN) 2004dj occurred within the young,
massive stellar cluster Sandage-96 in a spiral arm of NGC 2403. New
multi-wavelength observations obtained with several ground-based and
space-based telescopes are combined to study the radiation from Sandage-96
after SN 2004dj faded away. Sandage-96 started to dominate the flux in the
optical bands starting September 2006 (~800 d after explosion). The optical
fluxes are equal to the pre-explosion ones within the observational
uncertainties. An optical Keck spectrum obtained ~900 d after explosion shows
the dominant blue continuum from the cluster stars shortward of 6000 \AA as
well as strong SN nebular emission lines redward. The integrated spectral
energy distribution (SED) of the cluster has been extended into the ultraviolet
region by archival XMM-Newton and new Swift observations, and compared with
theoretical models. The outer parts of the cluster have been resolved by the
Hubble Space Telescope, allowing the construction of a color-magnitude diagram.
The fitting of the cluster SED with theoretical isochrones results in cluster
ages between 10--40 Myr, depending on metallicity and the model family. The
isochrone fitting indicates that the resolved part of the cluster has a bimodal
age distribution: a younger population at ~10--16 Myr, and an older one at
~32--100 Myr which is similar to the age distribution of the nearby field
stars. These stars may have been captured from the field during the cluster
formation. The young age of Sandage-96 suggest 12 < M_prog < 20 M_\odot as the
most probable mass range for the progenitor of SN 2004dj. This is consistent
with, but perhaps slightly higher than, most of the other Type II-plateau SN
progenitor masses determined so far.
