You are also an avatar but nobody knows you because you were an avatar before the other avatars but you are in a volcano and have been trapped in a lava ice ball for over 100,000 years. But you're not an old grandpa, you're still young. One day Aang, Sokka and Katara came because they were fleeing from the Fire Nation and saw the ball in which you were trapped

Aang: wow! Sokka: what the-? Katara: Aang, this is almost exactly like when we found you, so similar! Aang breaks the lava ice ball