Alahra hit the nail on the head. The system isn't about properly teaching someone up to level 60, or ensuring they know proper DRG rotation at 60 or ensuring they become a good <job>. It's about being a reliable resource for new players.

"I want to be a Ninja. Where do I find the NIN guild?"
"What stat do I put my points in for Thaumaturge?"
"Hey that gear is cool! Where can I get one?"
"I have to run [instance] for the story.Where do I find it?"
"I wanna make my own armor. What crafting job do I pick?"

These are more likely the questions that will be more frequently asked, and in answering them, you can establish a rapport with the newer player, making them a LOT more willing to listen later on when it comes to more intermediary/advanced gameplay aspects.

In areas where the game does not fully explain things or hold your hand, it's up to the community to take care of the rest. Being a mentor is about being a reliable, trustworthy resource for such occasions.