It is important to know how the degradation proceeds in a polymer used for electrical insulation in power apparatus, especially in severe environments. Sheets of flame-retardant crosslinked polyolefin (FR-XLPO) were aged thermally at 100, 135, and 155 °C with or without simultaneous irradiation of 60Co gamma rays. They were then analyzed experimentally. The samples aged purely thermally at 135 and 155 °C show more obvious changes in mechanical and dielectric properties than the other samples. While the elongation-at-break (EAB) first increases and then decreases drastically in a relatively early stage of aging, the indenter modulus (IM) shows almost no changes. After that, in a later stage of aging, EAB has no room to go down, whereas IM goes up to high values. It seems that the radiation-induced formation of cross-linked structures makes the polymer harder, which mitigates the progress of damages induced by the aging. Consequently, EAB can be a suitable indicator of the aging of FR-XLPO in a relatively early stage, while IM can be an indicator usable in a later stage.