- BACKGROUND: The age of onset of MS appears to influence the course of disease progression and people with younger age of onset might have a different disability trajectory. OBJECTIVES: To identify longitudinal patterns of disability progression, as measured by changes in the Multiple Sclerosis Functional Composite (MSFC), of young people in MS drug trials and to estimate the extent to which disability progression differ in two age groups (≤25 years and 26 - 35 years). METHODS: Data from the Multiple Sclerosis Outcomes Assessment Consortium (MSOAC) was used. Longitudinal patterns on the MSFC were identified using group-based trajectory models (GBTM). For difference between the expected and observed proportions of people with pediatric-onset MS chi-square statistic was used. Linear mixed models were used to estimate the average change in performance over time, age and sex. RESULTS: GBTM results showed little variability in performance over time. Mixed modeling showed that the younger group performed better for gait speed, dexterity, and cognition. Men performed poorer on dexterity and cognition. Distribution of people with pediatric-onset MS differed from expected on dexterity, cognition, and the EDSS. CONCLUSIONS: The combined use of trajectory models and linear mixed models provided rich information about the variability in function over time.