Perhaps the ghosts appear as how they see themselves - Since Anakin wouldn't have seen himself in a mirror for the best part of a couple of decades (or perhaps couldn't bear to look at himself), then the projection would be as a young man.
Obi and Yoda are used to their old appearance, so that's what their ghosts look like. Why don't they appear young? They probably can't remember what they look like young, having no holiday snaps to remind them
But is it that
big a deal?
Just my 2 cents