Background:~ Predicting ultimate lower extremity length is important in the treatment of lower limb length discrepancy (LLD), congenital limb deficiency, and other etiologies. Utilizing skeletal age over chronological age improves the prediction of ultimate lower extremity length. The recently described modified Fels knee skeletal maturity system allows for skeletal age estimation via imaging always available in LLD patients. We sought to compare the accuracy of the modified Fels knee skeletal maturity system versus chronological age in ultimate limb length prediction of a modern adolescent clinical population. Methods:~ The medical records of all patients treated at our institution over a 20-year period with unilateral lower extremity pathology and available lower extremity imaging before and after reaching skeletal maturity were reviewed. Skeletal maturity was defined radiographically by closed distal femoral, proximal tibial, and proximal fibular physes. The femoral, tibial, and lower extremity length was measured in all radiographs. The modified Fels knee skeletal maturity system was applied to all radiographs obtained before maturity to estimate skeletal age. The accuracy of 3 widely utilized lower extremity length prediction systems was compared when utilizing estimated Fels skeletal age versus chronological age inputs. Results:~ A total of 245 radiographs (109 before maturity) from 43 patients were eligible for inclusion. On cross-sectional analysis, linear modeling using Fels skeletal ages was uniformly associated with higher (improved) R2 values than chronological age-based models. On longitudinal analysis, skeletal age mixed-effects models had significantly lower (improved) Akaike information criterion and Bayesian information criterion values than chronological age models in all cases. Cohen d values were also significantly different (P$<$0.05) for the skeletal age models compared with chronological age models in all cases. Conclusions:~ In the treatment of LLD, the modified Fels knee skeletal maturity system can be readily applied to available imaging to improve the prediction of ultimate femoral, tibial, and lower extremity length. This skeletal maturity system may have significant utility in the estimation of ultimate LLD and determination of appropriate timing of epiphysiodesis. Level of Evidence:~ Level III.