Plato's Crito[1] deals directly with this question, after Socrates has been found guilty and sentenced to death for 'Corrupting the youth', his friends offer to help him leave the city rather than die.
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"
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.
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.