All kind of chemical reactions are time dependent, including photoxidations like this one.
Anyway and although not absolutely coinciding to the reality, photoxidations can be considered to have zeroth order kinetics under constant flux irradiance (or constant irradiation intensity, if irradiation surface is constant) and constant oxygen concentration (not a degassed solvent).
dC/dt = K → C = Kt
The rate constant K is temperature dependent, according to the Arrhenius equation.
Thus, the "actual" age of your samples at 80°C, can be estimated if the Arrhenius constant A and reaction activation energy Ea are known.
But even if the Arrhenius constant A and reaction activation energy Ea are unknown, they can easily be calculated by a set of a few measurements (say a couple) at different temperatures.
PS: If flux irradiance (or irradiation intensity) or oxygen concentration are not constant, your photoxidation kinetics is more complex.