Some responses to my post about age of consent and Romeo-and-Juliet laws have said something like this:
The question is not whether the behavior is harmful and wrong (in both cases, the answer is equally yes). Instead, it is whether someone is responsible. 16 year olds are not mature enough to decide to have sex; 30 year olds are. Thus a 30 year old can be punished for having sex with a minor, but a 16 year old can't. I don't really see the problem here.
This is an interesting theory, but I'm skeptical about it.
First, we generally think that 16-year-olds are supposed to comply with the law, are capable of doing so, and should be held responsible for failing to do so, especially as to offenses that are otherwise seen as serious crimes. We may sometimes punish them less severely, for instance by trying them as juvenile offenders. But we don't categorically let them off the hook for a wide range of crimes -- robbery, assault, drug use, and the like -- just because they're 16. (At some age, we might indeed let a child off the hook, or at least handle him through a system that is truly therapeutic and nonpunitive, but that age tends to be far below 16.)
Second, the "16-year-olds shouldn't be punished for what would otherwise be statutory rape because they're too immature" argument doesn't fit well the way many of these laws actually operate. Consider, for instance, the Florida law, which sets a general age of consent of 18, but allows under-24-year-olds to have sex with 16- and 17-year-olds. It's hard to say that the law is trying to cut slack to 21-year-olds because they're too immature, but treats 30-year-olds as criminals because they are mature enough to know better. Sure, 21-year-olds (and even 23-year-olds, who get the same immunity) sometimes are immature in lots of ways, but not generally in the law's eyes when it comes to criminal responsibility.
The same would be true if you had a law like Pennsylvania's "sex with 13-to-15-year-olds OK if the actor is no more than 4 years older than the 13-to-15-year-old" law, which is a pretty common sort of model. This means that even 19-year-olds are excused if they have sex with 15-year-olds (assuming a slightly under 4-year gap), but 18-year-olds aren't excused if they have sex with 13-year-olds.
There might be a good explanation for this rule -- I do think that sex with 13-year-olds is likely to be much more morally troubling than sex with 15-year-olds -- but concern that the 19-year-old isn't mature enough to be punished for having sex with the 15-year-old, though the 18-year-old is mature enough to be punished if the other person is 13, doesn't seem like a good explanation. The focus is on the age gap's causing the relationship to be somehow improper, not on whether the defendant is old enough to be held criminally responsible.