NeverarGreat said:

In many accounts of Near Death Experiences or OBEs, deceased loved ones are often seen as in the prime of their life, even if the OBE-er couldn't ever recall seeing them that way. So actually, having a young Anakin Skywalker ghost makes more sense than seeing the image of an old man who could have only appeared that way if he was not horribly mutilated decades ago. Of course, that would mean that the ghost of Obi-wan would need to be played by Ewan McGregor, and they would need to make a CGI version of Yoda of him in his vigorous tadpole years.

Those sort of anecdotes vary wildly, and of course there isn't any proof that the stories are true. However, mythology and religion are both well-documented with regard to the claims, and the hope to be "made whole" again in the afterlife is pretty much universal among people who believe in an afterlife.

In any event, the specific requirements of the movie need to be taken into account as well, i.e., the audience needs to recognize the ghostly characters, so it follows that the same actors who portrayed the characters in their natural life would portray them in their afterlife appearance.