Megan McArdle clinches it:
In my experience, the big dividing line is having kids. Read this interview with P.J. O’Rourke and discover some shocking things coming out of his mouth about how he doesn’t want his kids to do drugs. Having kids makes you realize how narrowly you escaped killing yourself–and remember all the friends who overdosed, or got arrested on a DUI, or spent their twenties working at a job that would let them smoke up three times a day, only to realize at age 35 that they had pushed themselves into a dead end.
Before the pot-smoking parents start crawling out of the woodwork to tell me that I’m totally wrong, that there are lots of parents who support legal marijuana–I’m not saying this happens to every single person who has a kid. But in my experience, as the kids approach the teenage years, a lot of parents do suddenly realize they aren’t that interested in legal marijuana any more, and also, that totally unjust 21-year-old drinking age is probably a very good idea.
I would add to this. Parents are not even preferring the policy which (necessarily) best protects their kids. Parents are preferring the policy which gives them a slightly better feeling of being in control of their kids, whether or not they are.