Here is a very interesting letter by a researcher named Jokin de Irala (and his co-writers) that is relevant to the "they're going to do it anyway" meme. De Irala et. al. compared statistics in three countries showing the mean and median ages of "first sexual intercourse" with their own findings as to the percentage of teens in those countries who are actually sexually active at those ages. And what strange things we do find. For example, we have the following odd situation in Spain: Mean age of first sexual encounter--16.3 years. Median--16 years. Yet Irala's research found only 21.7% of 16-year-olds were sexually active at all and only 34.8% of 17-year-olds. Other countries Irala checked turned up similar results.
How can these things be?
Irala suggests the following solution to the puzzle:
The mean age of first sexual intercourse was obviously estimated solely using subjects who have already had sexual intercourse whereas the proportion of youth that were sexually initiated use all youth in each age group as the denominator.
Ohhhh. I get it. So when we hear that the "mean age of first sexual encounter" is getting lower and lower, that just means among the young people who aren't remaining chaste during their teen years. That's a very different matter from "they're going to do it anyway." Let me repeat that another way: The data on mean and median age of first sexual encounter apparently do not include chaste teenagers! That's almost incredible.
As Irala calmly notes,
Sentences derived from average data such as this one—“compared with previous generations, young people (16–20 year-olds) were having intercourse for the first time at an earlier age, on average at 16.5 years of age” (Avery & Lazdane, 2008)—leave the facts as to how many from these age groups have, in fact, had sex unspecified. These confusing interpretations of epidemiological data create definite impressions that can be misleading and thus may hinder public health and educational interventions that are trying to delay sexual initiation in youth[.]
I don't know for certain whether researchers who report these mean and median ages were trying deliberately to mislead, but it will be interesting to see whether anything changes in the reportage now that Irala et. al. have issued this cautionary note. My bet: Probably not.