Many states have lower ages of consent for sex among minors than for sex between adults and minors. Thus, two 16-year-olds having sex may be legal, but not a 30-year-old having sex with a 16-year-old.
I share the intuition behind this distinction, but I wonder whether my intuition is right. For instance, I would think that quite a few 16-year-old girls who are interested in sex would rather be involved with 30-year-old men than with other 16-year-olds; the 30-year-olds are more likely to know what they're doing both sexually and romantically, plus are more likely to be much more emotionally mature as well as interesting to talk to. What's more, to the extent that women are said to be attracted in some measure to success (not just financial but professional and social), the 30-year-old may be much more attractive to them. Plus if a serious relationship ensues, the 30-year-old might be a better influence on the 16-year-old than another 16-year-old would be.
Ah, one might say, but perhaps the 30-year-olds are more likely to be sexually exploitive of the 16-year-olds, whatever "sexually exploitive" might mean. But why should we be so confident of that? Sixteen-year-old boys can be as interested as 30-year-olds in sexual conquests for the sake of sexual conquest, and can be as willing and able to lie and manipulate to get what they want. I suppose they might be less good at the lying and manipulating, for the same reason that they can be less good at some of the things the 16-year-old girl may want (being courted in a romantically appealing way). But I doubt that they're entirely unable to lie and manipulate -- and they may feel even more pressure to do so, because they may be more hormonally charged, sexually desperate, and desperate to prove their adulthood and manliness by getting sex or by racking up partners.
Of course, 16-year-olds are more likely to be thrown together with other 16-year-olds in social contexts, and are thus more likely to "naturally" become interested in each other. Perhaps then the rationale is simply that you can't stop such sex without prosecuting millions of people, while you can stop adult-adolescent sex, which might be more likely to be more common. But the effect of the law is still to channel some 16-year-old girls away from sex with adults and into sex with other teenagers. That would make sense, I think, only if we think that sex with teenagers is better for them than sex with adults would be. But why is that so?
Query, also, whether the analysis should be different when we get to 14-year-olds or 15-year-olds, but please set aside for purposes of the analysis flat rules that categorically forbid sex between anyone and someone under a certain age. Those rules are easy enough to explain, as being based on concern about (say) a 15-year-old not being mature enough to make a decision that can be so emotionally and physically dangerous. The tougher question is why the 15-year-old should be allowed to make such a decision when the partner is another 15-year-old as opposed to a 25-year-old. (Note that I'm talking about sex between adult men and adolescent girls; one can ask similar questions with the sexes reversed, and as to gay and lesbian sex as well. But my sense is that the bulk of the sexual conduct that is actually punished or deterred by these laws is between adult men and adolescent girls, so I thought I'd focus on that.)
Now perhaps my skepticism here is unjustified. I wouldn't mind being persuaded that it is unjustified, since as I said I find the distinction between "Romeo and Juliet" sex (the laws allowing sex between minors who are close together in age are often called "Romeo and Juliet" laws) and adult/teenager sex appealing -- though look what happened to Romeo and Juliet. I just wonder whether we can be confident enough in this distinction.
And, just to stave off the speculation, I've never dated a minor while I've been an adult, and have never wanted to date one. Among other things, I've generally found adult women considerably more attractive and interesting than underage ones (much as I suspect that some many adolescent girls would find adult men considerably more attractive and interesting than underage ones).