They shouldn't be allowed to vote beyond the local level, and they're volunteering for the military these days (which is as it should be). I'd be willing to extend voting privileges for active duty military personnel under 21, though, since they tend to have much more of a grip on reality than college students, most of whom are chronically inebriated and spend most of their sober time being brainwashed with PC garbage.
If they shouldn't be allowed to vote until 21, then they should not ever be allowed to be tried as adults.
All male Americans are required to this very day to register with the selective service at their local Post Office upon attaining age 18. I detect just a whiff of totalitarianism, and elitism in these posts. Women and Indians and Mexicans should likewise have voting and alcohol restrictions if this logic follows.