Polypropylene (PP) is a widely used common type of polymer with excellent chemical and mechanical properties. Thermal aging is a main culprit in degrading its lifetime and effectiveness. In this work, terahertz dielectric frequency response of PP during the process of thermal aging is monitored and analyzed to understand the underlying mechanism, and to qualify terahertz (THz) dielectric spectroscopy as a useful tool for time-stamping and benchmarking thermal aging of polymer materials. Using an improved THz material parameter extraction algorithm for thin films, the real and imaginary parts of the dielectric permittivity of standard PP test coupons were obtained at various stages of the thermal aging process. In agreement with theoretical analysis based on a molecular-level understanding of the thermal aging dynamics, the experimental results indicated that THz wave transmission loss of PP correlated quantitatively with the degree of its thermal aging.