You know what? Fuck it.
Yes, the USA exists as a legitimate entity. Because everybody 18 and over has the right to vote. That means that the will of those governing us comes from those who choose to exercise this right to vote.
You say, no, the right to vote doesn't equal the consent of the governed. I say, it's a compromise, Franc. You're making a semantic argument here, nothing more. The fact is that all government must come with compromise; it's the reason why neither pure communism nor precious anarchy could ever truly exist. As civilized people, we are generally benefitted by having a government. If we didn't have a strong and just government in place, who would be to stop some despot actually taking power and making life miserable for us all? And no, GW doesn't count. I'm talking about somebody with unlimited power. We'd probably revert to a tribal state if we didn't have a strong government.
And we have the right to help choose who governs us, and therefore what laws are enforced. It's not perfect, but do you really think perfect government exists, or can exist? Nothing is perfect. I think that it can be argued that the right to vote does equal the consent of the governed, because the way it's done is the most practicable way of putting such an ideal into effect.
Does it fit your technical definition, Franc? Probably not. Congratulations, Franc, you've managed to prove... absolutely nothing, actually.