Borrowing from this answer:
Kyoshi was born in 312 BG and died in 82 BG, living 230 years and is confirmed to be the longest living Avatar.
Avatar Kyoshi lived for 230 years before she died. How was she able to live that long, when as far as I am aware, the lifespans of humans in the Avatar universe is similar to real life?
Is there any canon explanation out there to explain how she achieved this? Word of God answers are acceptable, but speculation or theories are not.
In the novel "The rise of Kyoshi" there is some hint to how she lived so long. One of her first teachers was named "Tieguai the Immortal" and no one knows exactly how old he is. He tries to teach Kyoshi his technique of self maintaining one's mental and physical state. They don't really go into it more than that, and at least as far as the first book goes she doesnt fully belive her master is Immortal, but I imagine she masters it later in life. Here is the excerpt from the book that addresses it:
“So you’re telling me you’ve never wondered about my age?”
Now he was trying to goad her on purpose. It was astounding how easily he flipped from the hypnotic, terrifying vision she knew he could be into an oafish child with wrinkles and white hair. She was wrong to have thought that calling him sifu a few times would have given her consistent, uninterrupted access to a guru of death.
“I can’t say that I have,” Kyoshi muttered through her teeth.
He sounded slightly wounded by her lack of interest in his secrets. “It’s just . . . the people who’ve openly confronted me in the past with the name ‘Tieguai the Immortal’ . . . to a man, they all begged me for the secrets of longevity. The only ones who didn’t were you and your mother.”
First, she didn’t believe he was anywhere near as old as he claimed. And second, desperately grasping for more power and control over life was what people like Jianzhu did. Te too, probably.
“Sifu,” she drawled. “Oh, please, impart upon me the mysteries of immortality, for I wish to watch eras pass before my eyes like the grains of an hourglass.”
“Of course!” Lao Ge said brightly. “Anything for my dear student. You see, it all comes down to maintaining order. Keeping things neat, clean, and tidy.”
“Excuse me?” This was genuinely offensive to Kyoshi, as a former housekeeping servant. She’d let go of her standards for cleanliness the first morning outside of Yokoya, after waking up covered in Pengpeng’s shed fur. But with his drinking and aversion to changing clothes, Lao Ge toed the line of rancidity. What did he know about tidying up?
“Aging is really just your body falling apart, on the smallest, most invisible levels, and neglecting to put itself back together,” he said. “With the right mental focus, you could take an inventory of your own body and place each little piece that’s not where it should be back into the correct order.”
Kyoshi had to assume he was tailoring his lessons to her background and that the real process was much more complicated. “The way you describe it, you’d have to decide what version of yourself you’d be stuck as, forever.”
“Exactly! Those who grow, live and die. The stagnant pool is immortal, while the clear flowing river dies an uncountable number of deaths.”
“Is that another proverb of Shoken’s? Because it doesn’t sound like any spiritual lesson I’ve heard.”
“It’s my proverb,” Lao Ge whined, his feelings hurt again. “All this fretting about spirits. I’m trying to teach you about the mind. An infinite world that’s been neglected by far too many explorers.”
Hope that offers some explanation.
