We utilized time-resolved photoluminescence (PL) spectroscopy to investigate the photocarrier recombination dynamics in CH3NH3PbI3 thin films as a function of the time elapsed from the films fabrication. We found that the PL lifetime gradually increased and began to level out once the age of the film reached similar to 30 h. Even under weak excitation, the PL dynamics depended on the excitation intensity in the fresh sample, while the mature sample displayed no excitation-intensity dependence associated with the PL dynamics. We submit that this can be explained by the fact that a significant number of defects are initially formed in CH3NH3PbI3 thin films fabricated by the sequential method and are spontaneously reduced by room-temperature annealing. Our results provide important insights for reducing the nonradiative recombination centers, which improves the power conversion efficiency of perovskite solar cells.