I have more graphs to back this up, which I'll post soon, but for now let's just look at how approving people are of sex between teenagers who are between 14 and 16 years old. By disapproving, the person is saying that they're not adult enough -- that they should delay growing up, despite being capable of growing up now. It's like, "Yeah, you can work full-time right now -- but you should wait 5 to 10 years until you're through with your education." The GSS asks people the following question (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 that response and how it varies by the respondent's age, from 18 to 85 (click for better view):
No surprise that responses are highly predictable by age for a question about what a certain age group should or shouldn't do. The Spearman rank correlation is +0.96, two-tailed p less than 10^(-6). Those hotheaded 18 year-olds are the only demographic group I could find where a majority don't think teenage sex is always wrong. You can project the line to the left to see how the 14 to 16 year-olds themselves would respond. Hey, if the so-called victims don't think it's wrong...
This is just one case of the elders wanting to shield anatomically adult people from the harsh realities of Adult World, but there are similar trends for the proportion who think that people should get married young, should have children young, and so on. Older people don't want people passing any such milestones while young -- they'd prefer if young people delayed becoming adults -- while young people are eager to pass them sooner rather than later.
I'll also post a follow-up to this post at GNXP.com, looking at how disapproval of teen sex varies by four indicators of social class, and why it varies the way it does.