I was going to suggest that while all the others summonings are done with any body that Kabuto could "stock" and the dead were bind to a random body, (and so they look exactly when they died), Edo Mandara has his own body back, at least it's the idea i got, and it was enhanced/restored using HC cells to the point that he has the face in the chest.

So again, HC could prove to be another "counter" to any type of situation, but i do refute my own "idea" due to fact that the appearance of every edo tensei summoning has result in the person having the same one of when they died, why would Mandara be diferente even with HC ? It seems that what counts is the souls "age" not the body.

So i got to think in another direction, Mandara said that he awoke is rinnegan near his death ? Couldn't he have died in the process of awakening the eyes ? or very short time after, or even, due to some illness (like itachi), or any other factor ? that would mean that he didnt die of old age, that could explain the more youthful appearance of his soul/self.

Just another theory before today's chapter