To paraphrase, Socrates says "Though not explicitly, I have by my actions agreed to be ruled by the laws of Athens by carrying out my life here and not choosing to move away to somewhere with a different set of laws"
[1]http://www.columbia.edu/itc/lithum/wong/textclip.html @ [52b]
I believe Plato will say they consented to be governed by this state, not at birth, but throughout their life by choosing to stay in a place that is governed by this state and enjoying the benefits.