Alpine skiing performance depends on several qualities, including lower limb strength, power production and flexibility. However, how these physical skills, as well as their relationship, differ between sexes and evolve in growing skiing athletes is still unclear. We analysed sit-and-reach (SR) and three jump test (height and peak power) scores of over 200 alpine ski racers collected across 15 sports seasons. We stratified data among sex and age categories to assess differences and to interpret the relationships between flexibility and jump performance. The mixed models showed a significant age category and sex effect ( p < 0.001) for all the flexibility and jump performance scores, with males having higher jump scores but lower SR scores than females, and all performance scores increasing significantly through the age categories. The regressions between SR and the jump scores performed on the whole database were positively significant for the three jump variations ( p < 0.001, R²ranging from 8.9% to 15.9%), while when the relationship was reanalysed independently in each age–sex group, only some were significant. Both age and sex influenced the performance in the investigated lower limb power and flexibility qualities, along with their relationship. The presented results suggest that lower limb power and flexibility should be longitudinally assessed, as well as their relationship, for training and testing purposes, considering the sex and growth-related modifications.