That's a good question. Consenting to sex, consenting to enlistment in the military, signing contracts in general, assuming the rights (e.g., voting, driving, drinking, and so on) and responsibilities (e.g., abiding by laws and accepting punishments when one doesn't) as adults -- all require an ability to make judgments as adults. So, the logic of liberalism as political philosophy appears to dictate that the age of making judgments as adults be the same in all spheres of life.
-- Yoshie <http://montages.blogspot.com/> <http://mrzine.org> <http://monthlyreview.org/>