1) The state has authority over its citizens until the citizens reach a certain level of capacity. As an adult who makes most of his own decisions, I believe I have a level of capacity which allows me to make good decisions without a state. This seems reasonable, as the state has declared me "eligible to vote", and perform other tasks significant to the state, which appears to be an endorsement of my level of capacity.
2) The state is our eternal guardian, and has authority over all of us forever, thus North Korea and other totalitarian states have the moral authority to oppress and direct their people in ways that seem wrong, because we are but children.
The other problem with this reasoning is that it doesn't really define what gives a state this authority. If I declare my own state "The Kingdom of Nickff", and invade (a portion of) Canada, I have the same authority as the previous government, though I may be 'cruel' and 'despotic'.
Consider instead a different question - what authority your parents have over you when you're a child? After all, you haven't agreed on anything. Aren't they an oppressor, that often forces you to do things (like eating broccoli) against your will?