Yes, just like you can start collecting subjob quest items before you hit 18, some of the advanced job quests you can start early, or at least collect items needed. I did the first part of the Paladin quest at level 8 to get the Spatha :P
If you can handle acrophies, by all means go after that ruby, you just won't be able to actually do anything with it until you hit 30 and activate the Summoner quest.
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Samus taught us that a girl doesn't need brains to be successful. Brains are giant, evil and vulnerable to missiles.