i don't think anyone has done that research yet, g.
i would imagine that having optimal D3 status must be better than having suboptimal D3 status, regardless of age.
studies on D3 and MS:
READ ME key info on nutrient targets - www.thisisms.com/ftopict-2489.html
my approach: no meds so far - just nutrient-dense anti-inflammatory whole foods, and supplements where needed
info: www.whfoods.com, www.nutritiondata.com