The young people who have come forward to call for gun control in the wake of the mass shooting at their high school in Parkland, Fla., are challenging the tiresome stereotype of American kids as indolent narcissists whose brains have been addled by smartphones. They offer an inspiring example of thoughtful, eloquent protest. Unfortunately, when it comes to electing lawmakers whose decisions about gun control and other issues affect their lives, these high schoolers lack any real power. This needs to change: The federal voting age in the United States should be lowered from 18 to 16. Skeptics will no...