The most logical numbers for it, given that 1 and 50 have both explicitly been ruled out, are 15 and 30.
15 would skip the solo "tutorial" levels that really only take a few hours of play anyway, and send you straight into dungeons, which is one of the best ways to level up and gives you experience with other players that you will need for any job. The only thing you'd be bypassing here is essentially useless for anyone who has leveled anything at all in this game anyway.
30 would see consistency with the other "jobs" in the game, though that doesn't really mean much since these new ones don't operate off a base class anyway. In addition to the consistency, 30 is kind of right in the middle, where you're getting actual rotations and situational options coming into play, rather than just spamming the same few basic attacks.
Any other level would seem pretty arbitrary to me.