What's up with Sage Yulyana's lifespan?

Question

In Bravely Default, we learn why Lord DeRosso is immortal alright.

But what about Sage Yulyana? He is more than a thousand years old (but clearly he is aging) - why does Yulyana have such a long lifespan? Is it ever explained in the game?

Answer 1

It actually is explained in the game. You can find the explanation in the Notes section of D's Journal.

From Notes No. 14: "The Vampire Gallery: Archbishop" pgs 5-7:

But to do this, he would need time-- something he lacked after the long war with DeRosso. With the ornamental title of Archbishop came one material privilege: access to the head temple's treasury, and the writings sealed within it. There, he found a text-- The Rite of Agelessness-- and an arcane elixir of longevity beside it. Though incomplete, the sage supplemented the text with knowledge acquired in battlefields the world over. Even Lord DeRosso was shocked at how quickly Yulyana perfected the method, and how readily he used it on himself. He was 103, and an archbishop of the Orthodoxy; he would betray all expectations be remaining both.

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(but clearly he is aging)

I don't think that is actually the case. He is just really old when he becomes immortal, so he appears to be 103 indefinitely.