After defeating the Fire Lord at the end of Avatar: The Last Airbender, Aang continued to perform his duties as the Avatar and lived on for many years, but he’s not the oldest Avatar. Growing old is not something guaranteed for those who wield all four elements. The responsibilities of being the Avatar sometimes include facing dangerous opponents and risking an early demise. Many of the known Avatars appear to have died relatively young, with a few exceptions.
While Aang was physically 12 years old at the start of The Last Airbender, he died at 66 due to complications from being frozen for a century. Before Aang, Avatar Roku died at seventy, but others weren’t as lucky. Kuruk, the Avatar from the water tribe, succumbed at 33. Other Avatars who revealed themselves as spirits to Aang and Korra also appear as young adults, such as Yangchen and the first Avatar, Wan. It’s safe to say that the abilities of the Avatar don’t include a long lifespan. However, there is one major exception: Avatar Kyoshi. Not only she’s the oldest Avatar ever to live, but she’s also one of the oldest humans ever to live. Kyoshi was 230 years old when she died.
Kyoshi was a citizen of the Earth Kingdom and was born after Kuruk died. In the prequel novel The Rise of Kyoshi, we learn that she had a difficult childhood after being abandoned by her parents. In fact, at first she didn’t even know she was the Avatar. When she was younger, one of Kyoshi’s friends was misidentified as the Avatar instead of her. He was then told to train with an old master named Jianzhu. However, Jianzhu proved to be a cruel and controlling man, willing to do anything in the name of “balance.” Once he realized Kyoshi was the real Avatar, he killed her friend and chased her, so he could instill his teachings on her. Kyoshi fled and kept her identity secret, joining an opera company to keep a low profile.
Eventually, she confronted the master and revealed herself to the world as the Avatar. She would then grow to be a powerful and strong woman, capable of lifting a man with one hand. She also had no fear of confronting the elite of the Earth Kingdom, going against their corruption and greed multiple times during her life. She faced off against Chin the Conqueror and the Earth King, always putting the well-being of the populace ahead of their quests for power. She also trained the first Kyoshi Warriors and established the Dai-Li.
Kyoshi lived an incredible life, filled with great achievements - and one of those was becoming the oldest Avatar. It’s not clear how she managed to extend her lifespan for so long, but her appearance as a spirit suggests she was able to keep herself young. Perhaps she learned the secret of immortality from her spiritual master Lao Ge or Tieguai the Immortal. Lao Ge was an assassin who was hundreds or possibly thousands of years old by the time she meets him in The Rise of Kyoshi. Both of them might be the oldest humans in the Avatar: The Last Airbender’s universe.