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Background/Objectives Due to regulatory restrictions, children with multiple sclerosis (MS) often do not receive highly effective monoclonal antibodies until they reach adulthood. This study aimed to evaluate long-term disability in children with MS treated with monoclonal antibodies before age 18, compared to those who initiated these therapies in early adulthood.Method Patients younger than 18 years at the onset of MS symptoms were identified in three MS registries. Patients who commenced natalizumab, ocrelizumab, or rituximab between ages 12–17 into the early treatment group, and those who began treatment between ages 20–22 into the delayed treatment group. Inverse probability of treatment weighting was used to balance the treatment groups at age 18. The primary outcome was the change in Expanded Disability Status Scale (EDSS) scores between baseline and ages 23–27, evaluated using a Bayesian weighted generalized linear mixed model.Result Of the included 282 patients, 110 commenced high-efficacy therapy during childhood and 172 during adulthood. By age 23–27, EDSS scores increased slower by 0.57 steps in the early compared to the delayed treatment group (β=-0.57 [95%CrI: -0.90, -0.23]). This benefit was most pronounced within the EDSS range of 4.5–6.0, with up to a 97% reduction in the odds of further disability worsening (OR of an EDSS 5.0 over 4.5: 0.03 [95%CrI: 0.003, 0.23]).Conclusions Compared to commencing high-efficacy therapy in adulthood, its use during childhood is associated with reduced disability accrual at age 23 and beyond. Early use of high-efficacy therapy is crucial for preserving neurological function in children with MS.