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
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
“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.