Board Thread:Naruto Discussions/@comment-24824859-20150716001406/@comment-1018594-20150716030555

Probably similar to what happened with Nagato. Even though he was reanimated as crippled and decrepit, he was able to restore his body by absorbing enough chakra with the Rinnegan.

Madara was most likely reanimated as an old man, but was revitalized before being deployed. Like BeyondRed stated, Hashirama's face was Kabuto's modification. So perhaps Madara died with enough of his cells to awaken the Rinnegan, but not enough to use Wood Release.