In many cases, it was observed that service-exposed material delivers creep life comparable to unaged material. To understand the mechanism, a 1Cr-1Mo-¼ V casing casting steel has been given service-simulated aging treatment covering up to 150,000 hours. Room-temperature tensile strength, hardness, and fracture appearance transition temperature are evaluated for each time-temperature slot. Stress-rupture tests have been carried out following isostress methodology. Microstructural changes subsequent to aging and after stress rupture tests are evaluated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results have shown that, though there is a drop in mechanical properties of aged material, stress-rupture property behaved differently. Life steadily declines at intermediate simulation aging conditions up to 100,000 hours, but improves on further aging (≡ 150,000 hours). Thus, extrapolated rupture life at 530 °C (service temperature) produced a “C” shape plot in aging time vs rupture life. Metallographic observation revealed that, at intermediate aging condition, metastability and transitional behavior of alloy carbides are responsible for the drop in rupture life. On the contrary, improvement in rupture strength at the extreme aged condition (≡ 150,000 hours) is attributed to the stable carbides, viz. MC, M23C6, in-situ formation of (V, Mo)C (“H” type), and secondary precipitation of MC and M2C, in a ferrite matrix possessing relatively low density dislocation network.