First of all, this was simply a mindset change - he wasn't an entirely new personality. Otherwise, there would have been nothing to turn back to, would there?
Secondly, and this is the point everyone seems to miss: He becomes "Anakin Skywalker" again at the end of ROTJ. Thus, Anakin is as old as Anakin is at the end of ROTJ - unless you see his body getting younger as he is talking to Luke.
Point being, it's a stupid change.