I'm still going with my Sharingan hax immortality idea. Like..... How Madara dodged Naruto's attacks and Shino's bugs(I think? I forget.) by somehow teleporting a body part or some crap like that. So he is able to keep his whole body in that sort of state which is why he has never grown old or it just delays his aging.

Anyways, the only thing that still doens't make sense to me is obtaining the MS. There's too many ways to do it right now.