Background Surgical correction of pectus excavatum (SCOPE) is dependent upon chest wall pliability with optimal timing prior to complete skeletal maturation. Measures of skeletal maturity are not readily available for operative planning; therefore, surgeons use age as proxy despite patient-specific rates of skeletal maturation. We aimed to determine whether preoperative skeletal maturity is associated with postoperative pain as surrogate for chest wall pliability. Methods Children ≤18 years who underwent SCOPE from 2020 to 2022 were retrospectively identified. Preoperative CT within 3 months of procedure was reviewed by 2 radiologists and 1 surgeon. Skeletal maturity was determined by Schmeling-Kellinghaus classification which stages secondary epiphyseal ossification of the medial clavicle. Inter-rater reliability was evaluated. Schmeling-Kellinghaus stage and postoperative pain were compared. Results Of twenty-eight records reviewed, 57% were Schmeling-Kellinghaus stage 1. High inter-rater reliability was identified (inter-radiologist: kappa = .95, P < .001, all raters: kappa = .78, P < .001). Median age at operation was 15.5 years (interquartile range: 14.8-16.0) and increased with skeletal maturity ( P < .001). When comparing stage 1 (n = 16) to >1 (n = 12), stage 1 had lower maximum pain scores ( P < .001), total morphine equivalents ( P < .001), and benzodiazepine use ( P < .001) after surgery. Conclusions The Schmeling-Kellinghaus classification system is a valid proxy of skeletal maturity that can be applied with high inter-rater reliability. SCOPE during stage 1 was found to have less postoperative pain and narcotic use than more mature stages. This is proof of concept that skeletal maturity should be considered when determining optimal timing of surgical correction. Future research will evaluate the impact of skeletal maturity on postoperative outcomes.