What should be the legal age to purchase alcohol?

I think you should be able to drink at 18 if you are willing to give up guns, driving, and sex. Drinking and guns lead to shooting accidents which lead to costly emergency room visits and sometimes funerals. Drinking and driving leads to death or serious injury, most often for the pedestrian in the street or the people in the other car rather than the driver him/herself. Drunken sex leads to bad choices of sex partners, including choosing someone who is too drunk to consent to sex, and unwanted pregnancies. If you're willing to sacrifice those three things, drink all you want. If you violate those prohibitions, you get to have IDIOT tattooed on your forehead as a warning to others, in addition to any criminal consequences of your actions.