  The T2K experiment widely uses plastic scintillator as a target for neutrino
interactions and an active medium for the measurement of charged particles
produced in neutrino interactions at its near detector complex. Over 10 years
of operation the measured light yield recorded by the scintillator based
subsystems has been observed to degrade by 0.9--2.2\% per year. Extrapolation
of the degradation rate through to 2040 indicates the recorded light yield
should remain above the lower threshold used by the current reconstruction
algorithms for all subsystems. This will allow the near detectors to continue
contributing to important physics measurements during the T2K-II and
Hyper-Kamiokande eras. Additionally, work to disentangle the degradation of the
plastic scintillator and wavelength shifting fibres shows that the reduction in
light yield can be attributed to the ageing of the plastic scintillator.
