  Core-collapse supernovae (SNe) expand into a medium created by winds from the
pre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the
surrounding plasma, giving rise to thermal X-ray emission, which depends on the
density of the emitting material. Tracking the variation of the X-ray
luminosity over long periods of time thus allows for investigation of the
kinematics of the SN shock waves, the structure of the surrounding medium, and
the nature of the progenitor star. In this paper X-ray observations of five of
the oldest known X-ray supernovae - SN 1970G, SN 1968D, SN 1959D, SN 1957D and
SN 1941C - are analyzed, with the aim of reconstructing their light curves over
several decades. For those supernovae for which we can extract multi-epoch
data, the X-ray luminosity appears to decline with time, although with large
error bars. No increase in the X-ray emission from SN 1970G is found at later
epochs, contrary to previous reports. All five SNe show X-ray luminosities that
are of comparable magnitude. We compare the late-time X-ray luminosities of
these SNe to those of supernova remnants (SNRs) in the Galaxy which are a few
hundred years old, and find that when the tentative decline is taken into
account, the luminosity of the old SNe studied herein could fall below the
luminosity of some of the younger SNRs within a few hundred years. However, the
X-ray luminosity should begin to increase as the SNe expand in the Sedov phase,
thus reaching that of the observed SNRs.
