A considerable amount of expensive materials remain un-joined in selective laser sintering (SLS) additive manufacturing (AM) that generates complex metallic and high-performance polymeric parts from micrometer-diameter powders. Such materials, particularly the ones near the heat affected zone (HAZ), go through irreversible chemical degradations originated from thermal and laser-induced oxidations. In the SLS of polyamide 12 (PA12), despite efforts in understanding the degradation mechanisms of the materials, existing aging models center on thermal-induced oxidation. How to model fully material degradation in the complex SLS has remained not well understood. In this work, we propose a first-instance kinetic model considering both thermal and laser-induced oxidations to predict the degradation rate of polyamide 12 in SLS. By data-driven fitting of the laser-induced degradation into the kinetics of oxidation, the proposed model can predict the oxidation rates using two easily available parameters: materials density and oxidation time. The predicted oxidation matches on average 89.53% with results from experimentations, in contrast to a 34.48% accuracy from a conventional aging model. Furthermore, the new model applies to not only pure materials but also composite powders with mixed pure and recycled PA12 materials, making it adaptable to more complex material recycling configurations.