Polymeric insulation plays an irreplaceable role in a cable. Carbonyl index determined as the ratio of the infrared absorption around 1711 cm-1 due to carbonyl groups to a sample-dependent invariant reference band is often used to predict the lifetime or aging status of polymers. However, due to pre-existing carbonyl groups inside flame-retardant cross-linked polyethylene (FR-XLPE), it is not appropriate to use the carbonyl index to estimate its aging status. Instead, indenter modulus has come into our sight for showing a good monotonic change with the progress of aging. In this paper, sheet-shaped FR-XLPE samples were aged at 100, 135, and 155 °C for various aging periods from 800 to 2000 h. They were aged thermally or concurrently by heat and gamma-rays with a dose rate of 100 Gy/h. For these samples, various measurements were done to understand the aging behavior of FR-XLPE in different properties, including complex permittivity, conduction current, mid-infrared absorption, and indenter modulus. By analyzing the correlations among indenter modulus and other parameters, it has become clear that there are three different aging stages. At the first stage, cross-linking occurs in the sample, which causes a decrease in conduction current and the imaginary part of complex permittivity. However, when the aging enters the second stage, oxidative degradation becomes dominant, which gives a completely opposite change in these parameters. When the aging enters the last stage, the conduction current becomes stable, while the two parts of complex permittivity are still increasing.