"Every X Shall Have" = "Only X's Shall Have"?

A New Hampshire Supreme Court advisory opinion (handed down Monday) offers a fascinating case study in interpretation of legal texts. The New Hampshire Constitution provides,

All elections are to be free, and every inhabitant of the state of 18 years of age and upwards shall have an equal right to vote in any election.

The New Hampshire House of Representatives adopted a provision letting 17-year-olds vote in a primary if they'll be 18 by the time of the general election; it then asked the Supreme Court's opinion about whether this is constitutional. (Such advisory opinions are issued by many state supreme courts, though not by the federal courts.)

Unconstitutional, says the state supreme court: The state constitution "establishes the minimum voting age as eighteen years of age," and "the legislature has no authority to set a different minimum voting age." But, I ask, where is the minimum voting age here? The state constitution says that "every inhabitant of the state of 18 years of age and upwards shall have an equal right to vote in any election," but it doesn't say that only 18+-year-olds have such a right. It thus seems to me set forth a constitutional floor for who may qualify, but not a constitutional ceiling.

Maybe I'm mistaken on this; I can imagine that in some situations an "every X shall have" sentence implicitly means that "only X's shall have." But I don't see why this would be so here, whether based on the text, the context, or the constitutional history that the opinion recites. At least I would have expected the court to explain why "every" means "only," and I didn't see it doing so. Am I missing something here?

UPDATE: Commenter NH Atty points to an item that may support the court's decision -- I saw it when I first read the opinion, but perhaps I didn't give it enough credit: "The ballot question regarding age at voting submitted to the citizenry in 1976 reflected the convention's intent that the amendments to Part I, Article 11 and Part II, Article 28 lower the minimum voting age to eighteen. Question eight inquired, in pertinent part: 'Are you in favor of amending the Constitution to make the following changes relating to elections: (a) to reduce the minimum age of voters to eighteen.'" This might be the "constitutional history" evidence I was looking for in the preceding paragraph.

The trouble is that the constitutional text doesn't say "minimum age"; rather, it says that every 18-year-old shall be able to vote. So the question, which I wish the court had discussed, is to what extent the ballot materials offered the voters should trump the literal meaning of the text. But I do think that I didn't give the court enough credit for supporting its argument, given that the ballot materials do indeed support the "Every X" = "Only X" interpretation.