They have to start the job at a semi-reasonable level. By the time you're level 30 you've already unlocked a ton of quests, dungeons, ect, assuming new players can pick up the jobs right away, throwing them into level 30 content is not a good idea or making them grind another class to 30 is a little unreasonable.

I think they will start these guys off at around level 10-15. When ROG came out, you had to have at least a level 10 to unlock it. I could see them doing this again, but they also bump the starting level of these jobs to 10. I personally believe they might start the new jobs at 11 or 15, as level 11 is the first guildhest and 15 is the first dungeon. If they release the jobs at a higher level, their skills still need to sync down for FATEs, guildhests, and dungeons anyway. And unlike NIN, there's going to be 1 of each of the roles, so in theory the queues shouldn't be anywhere hear as horrific as DPS queues were when NIN was released.