Multiple sclerosis with childhood onset has been extensively studied recently, increasing the knowledge of the characteristics and long-term evolution of the disease in this age group. It is a rare condition accounting for less than 5% of all cases with multiple sclerosis. Exacerbating-remitting forms are, by far, the most common presentation at onset. The evolution to the secondary progressive phase as well as the assignment to irreversible disability landmarks take longer in patients with childhood onset compared with patients with adult onset, as shown in the KIDMUS study. However, patients with childhood onset reach these different critical phases of the disease at a younger age than patients with adult onset, therefore contradicting the notion of a more favorable prognosis in this age group. With respect to the pathophysiology of the disease, age at onset probably influences mainly the clinical phenotype of multiple sclerosis but not the underlying biological process, suggesting a similar pathophysiology of the disease whatever age at onset.