I'd honestly be against requiring level 50 for any full fledged Job unlocks, if for no other reason then because Jobs are a part of your character development, and shouldn't be held away from you for the entire journey to end game.

XI was the perfect example of this... it took 'forever' to get any of the classes beyond the base starting ones, so if you had one in particular that you just really liked and wanted to be, well, too bad for you.

R30 is a decent enough marker, however, R20 wouldn't be amiss either, as the sooner things of interest come into play, the better. Less chance of new players getting frustrated before they can be who they want to be and quitting, and you get to play with the Job elements for the larger portion of the game, rather then the lesser.

I honestly have no problem with some things unlocking at R50, such as special Job abilities or high powered aspects such as a Primal summoning(which likely won't exist, but you get the context), but don't lock out entire Jobs, they're (or they hopefully will be) a vast portion of many can enjoy about their character.

If you truly insist on withholding things till R50 for more 'hardcore' players, then still unlock all the primary Jobs earlier (R20/R30) and allow for Job 'promotions' at R50, similar to how the original FF did 'some' of their classes. White Mage -> White Wizard, Warrior -> Knight, Black Belt -> Master... that sort of thing. (Thief -> Ninja would be a 'very' bad idea however... too much difference between them).

That, however, would take more work on the Devs part, and perhaps an additional system, even though it, to me, would be even cooler to combine the traditional Job system with the original FF promotion system .