I had to vote "other" as well. If it were an unalienable right, then anyone could vote. I believe only adult citizens of those governments that have votes should be able to vote. Certainly so for the US. In fact I lean "anti-right" and would be fine with the voting (and drinking and military) age going back to 21. I think 25 would even be better.
I suppose some country could pop up with voting as a right for anyone, citizen or not, with no age restrictions. It would be an interesting experiment to watch from afar.
That's not how unalienable works. You have an unalienable right to life, liberty, and the pursuit of happinefs [sic]; but that doesn't mean that a 5-year old can go to the Supreme Court, when his parents send him to his room. It doesn't mean you can murder someone, be sentenced to imprisonment or death, and then claim a right to life and liberty. I'm not sure how people have handled these limits theoretically, but it seems to be the case that parents are basically entrusted with the unalienable rights of their children. Also, and forgive me if I use the wrong word here, but felons basically waive many of their rights. You want to carry a gun? Fine, but if you also decide to murder people with it, you've decided to give up most (or all) of your rights. Your decision; not ours.
Then we have to define what kind of right that voting is. I think I understand where you're coming from, but if we treat voting like the 2nd, then we should be against any voter ID laws, just like constitutional carry should be the law of the land.
Sorry to pick on you, but there's a reason why we talk about "civil rights." Some rights, like the right to defend yourself, are common to all humans, everywhere. Civil rights, like voting, you only have within your own society. I can't go to Jamaica on election day, and join in the voting, and come back home. I should be allowed to have a gun there, however. I would probably not be allowed to under their current law (whatever that is), but I should be.
As to the OP's question, I believe the Constitution has always included
Article IV, Section 4, which guarantees a Republican form of government to each state. That doesn't mean everyone gets the right to vote, but it does mean the state governments have to allow for representative government. "Republicanism" was seen as a radically egalitarian idea, in the old days.