It also depends if they are on a preferred server or not. they can actually level up way to fast and be above level 30 before they know it and not have completed that one slyph quest to start that part of MSQ that leads up to totok dungeon (cant remember the name of it). Without that particular MSQ completed all jobs are locked until then. THe game doesn't tell you this bit what so ever either there is no notice that it is available either. i hit level 35 and I haven't completed it yet. I had ot look it up to see what I was doing wrong. I was simply not far enough in the MSQ.

I will point this out, teh following dungeons are not dependent on the MSQ meaning you can unlock them just by being the appropriate level and talking ot the corresponding npc. Halatai, Quicksand Caves, Qarn, Darkhold, Aurum Vale.