Abstract: The age and size at maturation are crucial traits in an organism’s life cycle, influencing its growth, survival, and reproduction. While energy allocation models have been used to study growth and reproduction, they have not been directly applied to age and size at maturation. To investigate how energy allocation strategies affect age and weight at maturation, we propose a biphasic energy allocation model, focusing on pre-maturity and post-maturity, to understand the mechanisms of maturation and estimate the age and weight at maturation. This model is parameterized for female lake whitefish (Coregonus clupeaformis), and we perform parameter estimation, model selection, and sensitivity analysis. Our results indicate that lake whitefish allocate an almost consistent fraction of energy to growth throughout their lifetime and shift energy allocation from storage to reproduction at maturation. We find that (i) the weight at maturation is positively related to the fraction of energy allocated to growth; (ii) the age at maturation is highly sensitive to the coefficient and exponent in the energy assimilation rate; (iii) higher energy conversion efficiency to weight leads to earlier maturation at larger sizes. These findings suggest that energy allocation strategies cause variations in age and weight at maturation across populations. Additionally, our new insights into the ratio between asymptotic weight and mature weight across populations potentially resolve the discrepancies between the ratio's theoretical predictions and its empirical observations highlighted in previous studies.