The known theories of photon aging are discussed. A hypothesis of photon aging, based on the standard model of the expanding universe, is proposed, according to which photons emit subquants whose frequency depends on the time and velocity of their source motion. Formulas for the frequencies of the formed subquants and their degradation with time are derived. It is shown that over time, in the limit, they turn into particles with a mass equal to the photon mass, which was previously calculated by S.A. Nikolayev.