I haven’t done as much reading in ancient history as other periods, but I wouldn’t agree that pre-medieval societies didn’t have noble classes. There’s plenty of examples to the contrary, whether you’re talking about the Eupatridae of Athens or the patriciate of Rome or the Zhuhou of Zhou China.
I would hazard a guess that the main distinction between pre-medieval and medieval aristocracies is the relationship between the class and the state – whether aristocrats thought of the state as something that belonged to them collectively or an outside actor that should be interacted with through reciprocal agreements and otherwise resisted.