Your levelling will look something like this:
20-30: Stockades
30-44: SM
44-50: ZF
50-54: ST
54-58: BRD
58+: Strath

Now mind you, those level ranges are estimates, if you feel comfortable moving sooner, it can be done, just test the waters and see if you are ready for it. The best judge of moving to new content is getting the next rank of Arcane Explosion on your mages. You get it every 8 levels, starting at 14, so you'll be getting a new rank at the following levels: 14, 22, 30, 38, 46, 54.