The GSS asks people about the morality of premarital sex between post-pubescent minors (TEENSEX):
What if [a male and female] are in their early teens, say 14 to 16 years old? In that case, do you think sex relations before marriage are always wrong, almost always wrong, wrong only sometimes, or not wrong at all?
Most people say “always wrong,” so I’ll just look at those responses; the other responses have little room to vary since all must add up to 100%. How does opposition vary across demographic groups?  As for social class, the elites say they’re more ethical than the rabble, but on the other hand, they’re big supporters of “sex is natural and therefore can’t be harmful.” So which is it?
Let’s see what the data say:
Clearly, those with more prestigious jobs (SEI) are less opposed. Perhaps this is because the prestigious can get away with more shocking or outside-the-mainstream views. But that’s not what the three other measures of social class suggest.
As for real income (REALINC, in $5000 intervals), opposition increases from poor to lower-middle income people ($40K), and then declines somewhat steadily among middle and upper income people. My guess here is that middle and upper income people don’t think the teenage years count — as long as their kids get advanced degrees, make a lot of money, and don’t marry scumbags, they could care less if they fool around a little in high school.
Poor people, though, see their kids as living shorter lives — they enter Adult World sooner, so it matters what they do right after puberty. If you live in the same town you grew up in and plan to get married in your early 20s, having a reputation as a slut in high school will harm your prospects a lot more than if you move across the country five times before marrying, and even then only 10 years after graduating.
Education (EDUC) and intelligence (WORDSUM) show a similar pattern to job prestige and real income, as all are indicators of social class. Opposition increases up through 6th grade graduates, but declines pretty steadily among those who made it through middle school and beyond. For intelligence, like the other class variables, there’s an increase among the below-average, and then a steady decline among the above-average.
So, judging by these four ways of measuring social class, there seems to be a real difference between the upper, middle, and lower groups in how fast or slow they expect their children to grow up and do adult things. In biology jargon, this is a difference in life history strategies. There’s a parallel here between parents who invest more in their sons or their daughters, according to the Trivers-Willard Hypothesis: richer families favor their sons, while poorer families favor their daughters. One study found support for this idea by looking at how Gypsy vs. Hungarian families in Hungary invested in their children (free PDF here). Since lower class families care more about their daughters, they’ll freak out more if they engaged in premarital sex while still young and thus when reputations matter most. Upper class families will be thinking of their sons, though, and conclude, “Meh, let them sow their wild oats, as long as they settle down when the time comes.”
 It seems like younger people would be least opposed, since 14 to 16 is awfully close to their own age. They might think the police will come after them next. Sure enough, age is a very strong predictor here — indeed, the only demographic group I could find among whom a majority doesn’t consider it “always wrong” is 18 year-olds.