Original: Aang is depicted in the series as a vegetarian, as shown in "The King of Omashu" and "The Headband" when he refuses to eat meat since the Air Nomads taught him that all life is sacred.
Simplified: Aang is a vegetarian, as shown in "The King of Omashu" (the 5th episode of first season) and "The Headband" (the 2nd episode of third season) when he refuses to eat meat since the Air Nomads taught him that life is holy.