The BTI effect, which consists of both stress and recovery phases, poses a unique challenge to long-term aging prediction, because the degradation rate strongly depends on the stress pattern. Previous approaches usually resort to an average, constant stress waveform to simplify the situation. They are efficient, but fail to capture the reality, especially under dynamic voltage scaling (DVS) or in analog/mixed signal designs where the stress waveform is much more random. This paper presents a suite of solutions that enable aging simulation under all possible stress conditions. Key contributions include: (1) Compact modeling of BTI when the stress voltage is varying. The results to both reaction-diffusion (RD) and trapping/detrapping (TD) mechanisms are derived. (2) Efficient simulation under DVS, leveraging the new BTI models; (3) Silicon validation at 45nm and 65nm, at both device and circuit levels. As the results illustrate, it is necessary to combine both RD and TD mechanisms to accurately predict aging under changing stress voltages. Our proposed work provides a general and comprehensive solution to aging analysis under random stress patterns.