The first 6 Jobs available (Blm, Whm, Rdm, Monk, Thf, Warrior) really should be unlocked as soon as you unlock the system, which I would think shouldnt even be accessable until you hit 20. So even if you have the skills unlocked if you don't have a rank 20 you don't even have the system unlocked. Then the more skills a job needs to unlocked they higher it can go, But no Job should require higher then rank 30 on any class to unlock.