I can see where this is confusing you/upsetting you. If you would give me but a moment of your time.

If you were a employed as a Grade School Teacher, you'd most likely want to start with year 2 or year 3 students. This is because they respect their elders. Generally speaking. You aren't experienced enough to teach hormonal teens yet.

Now at some point in your career; you might be offered a job. (key note; "job" reference) This job will require a summer course of sorts before you can become a teacher of higher learning. (University and so on) By this time, you'll have taught children from ages 5 to 15. Perhaps even 18. Now then, teaching is pretty much the same during every phase.

The application of skills change but the theory is the same. In FFXIV the same applies to your "classes" and "jobs" Conjurers will heal and will continue to heal as White Mage. Thaumaturge will continue to deal magic based damage as Black Mage.

But before you can become a White Mage or a Black Mage, or even a University Professor. You have to take supplemental classes to further your knowledge of said roles. In other words. Job Quests starting at level 30.

I hope this helps explain. Have a nice day.