Jax's bio -- PERFECTION
It addresses big questions without giving too much away.
The fire in his lamppost gives him unknown extended life, but his natural lifespan is also still very, very long.
"As he bore the elemental fire, Jax’s life extended beyond even the expectations of his long-lived people."
Jax's father was alive during the fall of Icathia which means Jax was born after, but considering the long lifespans there's not much room to speculate on the exact time of birth before Icathia's fall save the time he spent becoming a weapons master and gaining the experience he has.
They triumphed in early engagements, and Saijax even killed one of the vaunted Ascended in battle, watching with pride as its corpse was paraded around the liberated city of Bai-Zhek.
This quote seems to suggest that he indeed did beat an ascended in one-on-one combat, and a very strong one at that.
He became a wanderer, traveling across the known world and to places beyond any map.
Jax is one of if not the only canon champion to have explored the world.
