He is 16 during ATLA. When Iroh is telling the story of the Agni Kai between Ozai and Zuko he says it happened 3 years zamani when Zuko was 13. He is STILL 16 kwa the end of the onyesha since the the onyesha basically starts at the Winter Solstice (when Aang gets his message from Roku at the island) and ends at the end of Summer when the Comet returns.