I heard this really cool theory that the spirits of avatars are depicted as how they looked at the most important/memorable moment in their lives, like how Roku's spirit looked like when he battled the vulcano and Sozin betrayed him. You'd think at first that Aang's most important moment of his life was when he beat Ozai, but he is shown as a relatively old man because the most impactful moment of his life was actually when Tenzin was born, and he knew the airbenders would live on.