  Hadronic $\gamma$-ray sources associated with supernova remnants (SNRs) can
serve as stopwatches for the escape of cosmic rays from SNRs, which gradually
develops from highest-energy particles to lowest-energy particles with time. In
this work, we analyze the 13.7~yr \emph{Fermi}-LAT data to investigate the
$\gamma$-ray feature in/around the SNR G298.6$-$0.0 region. With $\gamma$-ray
spatial analyses, we detect three point-like components. Among them, Src-NE is
at the eastern SNR shell, and Src-NW is adjacent to the western edge of this
SNR. Src-NE and Src-NW demonstrate spectral breaks at energies around/below
1.8~GeV, suggesting an old SNR age of $>$10~kyr. We also look into the X-ray
emission from the G298.6$-$0.0 region, with the Chandra-ACIS data. We detected
an extended keV source having a centrally filled structure inside the radio
shell. The X-ray spectra are well fit by a model which assumes a collisional
ionisation equilibrium of the thermal plasma, further supporting an old SNR
age. Based on our analyses of the NANTEN CO- and ATCA-Parkes HI-line data, we
determined a kinematic distance of $\sim$10.1~kpc from us to G298.6$-$0.0. This
distance entails a large physical radius of the SNR of $\sim$15.5~pc, which is
an additional evidence for an old age of $>$10~kyr. Besides, the CO data cube
enables us to three-dimensionally locate the molecular clouds (MCs) which are
potentially interacting with SNR G298.6$-$0.0 and could account for the
hadronic $\gamma$-rays detected at Src-NE or Src-NW. Furthermore, the
multiwavelength observational properties unanimously imply that the SNR--MC
interaction occurs mainly in the northeast direction.
