One relatively simple (but also easily gameable) criteria is education and/or intelligence. Only 18-year-olds with a high school/college/postgraduate degree, only 18-year-olds with an IQ score/SAT score >= X, etc. We don't want to try that because we know how quickly the tests and measurements would be twisted with ideology, and we worry that we would end up systematically discriminating against a class of people based on some hidden criterion other than intelligence/education, such as political views.