The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account of age. Congress shall have the power to enforce this law through appropriate legislation.
While this amendment represented, at the time, an important recognition that citizens should not be subject to being drafted into a war by a government which was unaccountable to them, it largely ignores the larger issues involved with the existence of a voting age.
The United States government style is described as a representative democracy, and that says something about the nature of what it means to be granted the vote. The entire point of sending representatives to make decisions on our behalf rests on the idea that citizens aren't able to spend the needed time to educate themselves on every issue of significance to them. So instead, they appoint someone who's sole job is to become educated on these issues, and to make decisions in the place of the citizens who they are representing. The representative is intended to be someone who's judgment the citizens trust, as that representative is, ideally, acting on what the citizen would be acting on had the citizen had the time to properly review an issue.
I felt it important to go over the basics of representative democracy because I feel it's important to a common argument for why people under 18 ought to be denied the right to vote. The common argument is that people under 18 aren't educated or informed enough to have sufficiently developed political opinions on the relevant issues of the day.
The problem with this reasoning, is that, as I outline above, the whole point of having a representative rather than a direct democracy is that we already assume the voters are uninformed. It is an implicit assumption written into our form of government. The governing system is explicitly designed to handle uninformed voters. That's the whole point of it.
Another common argument for denying the vote to individuals under the age of 18 is that they are too strongly influenced by their parents, and this would effectively be giving parents more votes than the childfree, and parents of larger families more votes than those who have smaller families. On that front, I submit that the problem doesn't exactly go away when someone crosses the magic age line.
Individuals under the age of 18 are subject to the laws of this country. They are required to pay taxes on any income earned, and they are punished by the legal system when they violate laws made by individuals they had no part in electing. This is an immoral state of affairs, and one rather important American war has been fought over the very issue of taxation and legal responsibility without representation.
The 26th amendment forbids states from denying people over 18 the right to vote, but that is a minimum standard. States are free to grant the vote to minors as they see fit.
Suffrage has taken a long, difficult road to reach its current levels, gradually including non-land-owners, blacks, women, but we've still got entire groups of people denied basic representation. The way our representative democracy is supposed to work, a person deserves the right to political representation once they've reached the point that they can articulate an opinion, and age should not be a factor.