The reliable operation of power transformers is paramount to the stability of modern electrical grids, which are increasingly stressed by volatile loads from electric vehicle charging and intermittent renewable generation. Accurate thermal monitoring and aging estimation of transformer insulation are governed by two principal international standards: the International Electrotechnical Commission (IEC) 60076-7 and the Institute of Electrical and Electronics Engineers (IEEE) C57.91. This study presents a detailed parametric comparison of these standards through a sophisticated MATLAB-based simulation framework. The analysis systematically evaluates differences in physical thermal models, insulation aging mathematics, and their application across multi-stage cooling modes—Oil Natural Air Natural (ONAN), Oil Natural Air Forced (ONAF), and Oil Forced Air Forced (OFAF). A critical finding is the identification of a "reversal of conservatism," where the IEC model predicts higher hot-spot temperatures under static (cooling failure) conditions, while the IEEE model is more conservative during normal operation with active cooling. The study quantifies the catastrophic impact of cooling system failures, demonstrating a four- to five-fold multiplier on insulation life-loss during overloads. Furthermore, two-dimensional operational "heat maps" are developed to delineate safe operating zones, providing utility operators with a practical tool for risk assessment under demanding loading schemes. The results conclusively demonstrate that the standards are not interchangeable and that for modern transformers using Thermally Upgraded paper, divergence stems primarily from the physical thermal models rather than aging mathematics. These insights are crucial for ensuring transformer reliability, optimizing loading practices, and informing future standard development in the context of evolving grid dynamics.