A Force ghost does not have a physical reality. The way it appears to a living Jedi is a combination of the way the Jedi chooses to see it and the way the dead Jedi chooses to present himself. Anakin appears to Luke as his 22 year old self, the way he appeared just before he got burned up. This is the way he wants to be seen by Luke. He has been moved by the concept of his own metaphorical death, that little zinger Luke hit him with just before they went to see the Emperor.