My final Volkh entry that I've been saving for more comment gives some helpful criteria for figuring out when it might be worth having laws against certain behavior for children under a certain age, distinguishing between reasons for having different ages for smoking, alchohol, and sex.
What really struck me as odd wasn't Volokh's claim that people would be more resistant to restricting their sexual activity at age 16, say, than they would if you restricted their smoking or drinking activity. I think a good case can be made that the biological drive for sex is much stronger than any motivation to start smoking or drinking alcohol would be (before actually becoming addicted, anyway). That doesn't mean we have some need for sexual interaction that makes our lives somehow ridiculously impoverished without it. That's an insult to the fulfilled lives of many people who remain single and celibate their whole lives (not to mention those whose sex lives are no less rich simply because they delayed sex longer than some of their peers, waiting until getting married). Still, that wasn't what really caught my eye. What struck me as most odd was was his assumption without argument that someone at age 16 is mature enough to have sex. His whole argument for allowing someone to take pictures of herself having sex depended on her being mature enough to have sex to begin with.
Enough evidence exists now to show that the part of the brain affecting moral decision-making isn't mature until something like age 24, with a large part of its maturation taking place during the few years before that age. I don't have any links to this research, but it was a cover story in one of the major weekly news magazines a couple years ago. In the light of that, I think it's a bit presumptuous even to say that a college student is old enough to consent to sex. With all the peer pressure factors, together with the impulsive impetuousness of youth, I would suspect any college student's moral reasonings in sexual matters without first doing a careful evaluation of their moral maturity. Some are more mature than others. My experience in teaching on ethical issues related to sex in ethics courses has given me a lot of evidence that many college students are not morally mature enough to make a rational decision about whether sex in certain contexts is morally ok. There are so many issues, I've discovered, where someone who hasn't entered into a long-term commitment to someone, who hasn't had responsibility for the lives of young children, who hasn't owned property, and so on hasn't understood so much of why things that seem restrictive and puritanical can sometimes be for the best. There are the few rare people who understand this as teenagers, but most don't. Most college students over 20 don't. Some of my graduate student colleagues in their late 20s don't.
This doesn't mean we should raise the age of consent to 24. Volokh's other considerations do come into play here, and I'm not going to get into all those (go read it). However, it does seem to me to be at least tendentious to claim that a 16-year-old is morally mature enough to consent to sex.