I keep changing my mind on this, ranging from age 15 to 25 over the years. Overall, I think voting is so plagued by irrationality (note, I don't define that as voting different from me, I can understand reasons for voting different and consider it rational or understandable based on their perspective) that I find myself apathetic on the issue. Just the one thing I'd do is align voting age with all other age of majority (that includes decisions about sex, alcohol, driving, marriage, tobacco, etc).
In a more ideal world I'd like it so that people had to take a test to prove they at least read their voter's guides and knew the candidates they were voting for and against before they voted (in which case I'd be much more open to lowering the voting age), but I'm absolutely certain that the process would be corrupted if it didn't start out as corrupted to disenfranchise certain demographics and rig the elections. Politics is arguably where we humans play the dirtiest (and people may even feel guilty for not playing dirty, thinking the ends justify the ignoble means, in addition to the run of the mill unscrupulous for power and/or profit and/or ego). So I have to regretfully let that idea go.
I'd settle for teaching HOW to think than WHAT to think in schools for the kids (which has not existed in US schools for decades now, and is disappearing from our college campuses as well), and then maybe lowering the voting age (in which case I might have more confidence in their votes than in many of their elders). Even as it is, there are many kids whose intellects I respect more than many of their elders (of course they're not stuck in a bubble with all their time managed by adults--even then I can see potential in some if only they were allowed to develop it).
It would also be quite educational if kids learned how school boards got elected and the reasons for their policies (a dirty microcosm of the macrocosm in US politics), and if it didn't turn them off politics completely then it might make them much more discerning voters and political activists that we need (though the reporters also need watchdogs watching them). Politics affect them as well.