Thermal and structural properties of aged silicone rubber composite insulators widely used in high-voltage transmission power systems were characterized by photothermal radiometry (PTR) to evaluate the degree of aging. A two-layer PTR theoretical model was developed to extract the thermal diffusivity and the aged layer thickness from the PTR frequency-scan data. Results show that aging generated an aged layer with a reduced thermal diffusivity at the surface of the composite insulator. Further investigation into the depth dependence of the thermal diffusivity of aged composite insulators found a monotonically increasing thermal diffusivity along the depth direction, approximating to a hyperbolic tangential profile, which indicates a spatially continuous variation from surface to bulk during the aging process. The PTR depth-profiling of thermal and structural properties provides a deeper understanding of the aging process in composite insulators.