Indeed, I wouldn't object to this directly. One could however argue that it is bad for indirect reasons. It would acquire huge administrative efforts to test teens for their competence at voting, and the money and resources might be better spent on education or the US army (jk). In order to save administrative costs, using a Schelling point at the age of, say, 18, makes perfect sense, even though there certainly is no magical change taking place in people's brains the night of their 18th birthday.