The Sage job change is the second advanced class of the Mage line. A Sage blends magic with scholarship: studying elements, reading an enemy's gear, and striking with elemental weapon attacks. To become one you enroll at the Schweicherbil Magic Academy in Juno, pass a written entrance exam, complete practical classes with the professors, and write your own dissertation. You need Mage job level 40+. No pay-to-win here — just knowledge and a handful of gathered items.
Dean of the Academy (Kayron Grik) near 38,61 — he explains the Sage path and sends you to apply at the academy.
Old Magicbook and 1 
Necklace of Wisdom (2 of each across the registration courses).
Feather of Birds, 
Animal Skin, 
Trunk, 
Squid Ink, and 
Empty Bottle — these become your writing tools and you draft a thesis.
Stone — some are transmuted into elemental ores (
Crystal Blue, 
Green Live, 
Wind of Verdure).
Crystal Arrow, 
Stone Arrow, and Wind Arrow — the class covers the elemental strength/weakness chart.
Holy Water, then gather writing supplies again (Feather of Birds, Animal Skin, Trunk, Squid Ink, Empty Bottle) to write the next thesis; completing the courses grants you a 
Book (your dissertation).Dean of the Academy with the finished dissertation (the Book in your bag). Make sure you are job level 40+ and have spent all skill points — otherwise he asks you to sort that out first.Book as a memento.