Please do not argue that Colorado’s law, like those proposed elsewhere, bans sales to those under 21. Ha! I have teenage children. The laws against underage drinking represent more challenge to overcome than barrier to access.

And although alcohol seems to be the teen drug of choice among the adolescents I know, the more widely available marijuana becomes, the more minors will use it. If seniors in fraternities can legally buy pot, more freshmen and sophomores will be smoking more of it.

And it’s not as if the kids need encouragement. By the time they have graduated from high school, nearly half have tried smoking pot; 16.5 percent of eighth-graders have. More alarming, the number who perceive great risk from regular use has been plummeting, from 58 percent to 40 percent among 12th-graders, according to a study funded by the National Institute on Drug Abuse.

And, that study says, for those who trumpet tight controls on sales to minors, a third of 12th-graders who live in states with medical marijuana and who have used the drug in the past year report that one source is another person’s prescription. Another 6 percent have their own Rx.