This was from a study (2006?) by the American Academy of Neurology:

Conclusions: Late-onset multiple sclerosis (LOMS) is not necessarily associated with a worse outcome: first, progression in the primary progressive or relapsing patients differed little between late-onset vs adult-onset; secondly, those with LOMS were older when reaching Expanded Disability Status Scale 6. The disease course has a far greater implication for disease prognosis than the presence of LOMS.