Read more from Slate's Sex Issue.
Steinberg concludes that "risk-taking increases between childhood and adolescence as a result of changes around the time of puberty in the brain's socio-emotional system." In tests, these tendencies peak from ages 13 to 16. Subsequently, "[r]isk-taking declines between adolescence and adulthood because of changes in the brain's cognitive control system—changes which improve individuals' capacity for self-regulation." The latter kind of competence doesn't reach adult levels until the mid-20s.
Lay out these numbers on a timeline, and you have the beginnings of a logical scheme for regulating teen sex. First comes the age at which your brain wants sex and your body signals to others that you're ready for it. Then comes the age of cognitive competence. Then comes the age of emotional competence. Each of these thresholds should affect our expectations, and the expectations should apply to the older party in a relationship as well as to the younger one. The older you get, the higher the standard to which you should be held responsible.
The lowest standard is whether the partner you're targeting is sexually developed as an object. If her body is childlike, you're seriously twisted. But if it's womanly, and you're too young to think straight, maybe we'll cut you some slack.
The next standard is whether your target is intellectually developed as a subject. We're not talking about her body anymore; we're talking about her mind. When you were younger, we cut you slack for thinking only about boobs. But now we expect you to think about whether she's old enough to judge the physical and emotional risks of messing around. The same standards apply, in reverse, if you're a woman.
It's possible that you'll think about these things but fail to restrain yourself. If you're emotionally immature, we'll take that into consideration. But once you cross the third line, the age of self-regulatory competence, we'll throw the book at you.
What do "cutting slack" and "throwing the book" mean? If you're young, we could let your parents handle it. We could assign a social service agency to check up on you. We could require you to get counseling. We could issue a restraining order. We could put you on probation. We could put you in a juvenile facility, a mental institution, or jail. In the worst case, we could subject you to a mandatory minimum sentence.
Whatever the particulars, the measures taken should be developmentally appropriate. "Age-span" provisions, which currently allow for sex with somebody near your own age, are a good start, but they're not objectively grounded. That's why they differ wildly from state to state. I'd draw the object line at 12, the cognitive line at 16, and the self-regulatory line at 25. I'd lock up anyone who went after a 5-year-old. I'd come down hard on a 38-year-old who married a 15-year-old. And if I ran a college, I'd discipline professors for sleeping with freshmen. When you're 35, "she's legal" isn't good enough.
What I wouldn't do is slap a mandatory sentence on a 17-year-old, even if his nominal girlfriend were 12. I know the idea of sex at that age is hard to stomach. I wish our sexual, cognitive, and emotional maturation converged in a magic moment we could call the age of consent. But they don't.