To unlock this job level up a Magician and a Cleric to level 25 then head into the Church to create a Sage Star Child.
This should be doable by that point using certain maidens but the following minimum stat requirements for Sage Star Child creation need to be met:
ATK 166
TEC 156
DEF 164
SPD 135
MAT 181
MDF 176
LUC 106
If unable to make a Sage, continue to deepen Bonds with the Star Maidens until you can.