I do not understand why people say that obito's right eye, is still capable of being used. If I'm not mistaken, Obito's right side of the body, was destroyed. Face included. I.E. his sharingan. However, if this were possible, then I suppose Tobi is actually Madara Uchiha. While Danzou, is Izuna Uchiha, Madara's younger brother, who has Obito's eye, and possibly his body. O.o
Possibly, Madara somehow got his hands on Obito's body, granting Izuna 'light' once more. Due to transplant of course (Of eye, and body). I say that this explains it, sort of. Danzou's right side of his body the same as Obito's...? Yeah, I see a similarity. We should all know, that Madara is immortal, so he may seem young, I guess. While Danzou, who may have Obito's body, is getting old. Eh...
Also, the age difference between Danzou, and Sarutobi. Danzou is currently 72 years old, while the late Sarutobi was 69. If I recall correctly, at the time when Izuna and Madara owned the Uchiha clan, Sarutobi was either a mere child, or was not born at all. (70 years before series started) How the battle between Sarutobi and Danzou happened...? I do not know.
But how could this transfer of bodies, and stuff occur...? Pein!
That's what I think. >.>