To me, "young Skywalker" is far more personal, and implies that Luke is so well known across the galaxy that you don't even need to use his first name. It makes it far less impressive that the Emperor is giving this information to Vader. The Emperor is this mysterious figure at this point, and his knowledge of Luke is a sign of his power and influence.
Reporting "Luke Skywalker" conveys this in a far more meaningful way.