The global warming and pollution issues have reached an alarming level that requires immediate actions by all governments in the world to reduce fossil fuel vehicles, and even ban them in the foreseeable future. As a result, electric vehicles (EVs) are gaining ground rapidly. Inside the EVs, especially their power train, semiconductor power devices are considered the key components. Increasing the performance, energy efficiency, and reliability of power MOSFET, therefore, is critical in the EV industry. In this work, we stress the reliability and lifetime of power MOSFET devices and propose an aging model for them. We will show the procedure to construct the efficient aging model for a power MOSFET, supported by circuit-level simulation results. The aging conditions, including high temperature and/or high voltage on the gate oxide, will result in threshold voltage shift, which in turn decreases the MOSFET's switching speed and driving capability. With the proposed aging assessment tool for power MOSFET, the device's performance under different aging conditions can be predicted. The experiment results show that aging will reduces its switching speed and driving power. Experimental results by our tool also show that the circuit efficiency, ripple voltage, switching loss, etc., are negatively affected by aging.