Yes. Retainer level is based on how far you have levelled that class yourself, so if you've never actually bothered levelling a specific class but just have it unlocked (in other words, it's still just level 1), and then switch your retainer to it, they will never gain exp in it or level up until you level it yourself.

At the moment you also cannot equip Jobs to your retainers (meaning they are stuck as base classes), but that will apparently be changed in a future patch from what I am aware (hence this means you cannot at the moment set a retainer as a DRK, MCH or AST either).

Anyway, welcome to Eorzea!