I'm all for sufrage of those 18 years or older...with a few stipulations.
The Amendment should read thus:
Section 1. 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.
Section 2. Each citizen shall satisfactorily complete a college-level Political Science and Economics course, or possess a Bachelor's degree from an accredited institution of higher education, or be currently serving or have served in the United States Military in order to achieve voting status.
Section 3. The Congress shall have the power to enforce this article by appropriate legislation.
Why such regulations on who can vote? Good question. When this Republic was founded, voting was restricted to those citizens who owned land and/or paid a certain amount in taxes. The reason being that only citizens who had a legitimate stake in society had the right to decide who would lead.
My current rationale is analogous. Only people that understand the basic US political structure, as well as the foundations of our economic system should be allowed to decide who should lead.
I mean really, if you don't understand that both McCain's and Obama's platforms spell certain economic doom for the next 4 years, you shouldn't vote (see here for non-partisan analysis). Unless of course you understand that, and still think their platforms are beneficial, then you can still vote. Either way, you ought to have some grasp on what the implications of their platforms are before you vote.
Since there is no way to ensure that every voting American actually understands what they are voting for, an arbitrary yet beneficial requirement like a couple college courses, a degree, or military service seems like a great way to enforce said ideal.
"What nerve," you might exclaim. We already do this in our country with driver's licenses. Yep, some arbitrary written and practical requirements, while seemingly useless for some, are arguably very beneficial for the whole of society. I think you'd see the same results out of a similar voter's license.
My proposed amendment wouldn't discriminate, and there'd have to be a provision for all pre-18 year olds to take the required courses. We'd all be smarter, more informed voters voting for the same stupid people.