There's really no way to come up with a formula that determines a person 18 years 0 days old is "qualified" to vote while another person 17 years 364 days old, is not. It ain't like the Vote Fairy descends with a magic wand of flooby dust on one's 18th birthday.
In truth some people are knowledgeable and qualified at 18 or much younger while others expend their entire life never attaining that state at all. Which is why the Duopoly party dumbs everything down to catchphrased pandering.
What should really happen is we develop a civics test that determines whether you know enough about how things work in order to vote. Regardless of age. I'll wager we'd lose a hell of a lot more voters than we'd gain.
I've talked to lots of people who can't tell you how many people are in the House of Representatives or why, or how many judges are on the Supreme Court or what kind of cases they hear, but they can tell you how they feel about immigration, abortion, gun rights, etc.
I don't agree with a civics test, although I teach the rudiments of it to our Hi-SET students. The more you know, the better. But the laws apply to all, so all should have a chance to voice their choice.
Didn't see this post before. This is some waters I'm just treading out to thinking out loud. Clearly there is no magical ability that descends on one by virtue of attaining the age of 18, that wasn't there yesterday, so clearly that's not a legitimate standard. And just as clearly there walk among us (and post among us here) those who are twice, three times, four times that age who haven't a clue what they're doing yet get qualified to vote by the same virtue of having attained X number of years.
So I must conclude that clearly the present benchmark is inadequate and some new one must be sought, else we stay in the same place.
Can't find the clip right now but I refer you to that infamous clip of a voter in West Virginia who says in 2008, "I've had enough of Hussein!". We can all come up with 16 year olds or 14 year olds who know better than that.