A kind person has pointed me (via a locked entry in her own lj) to the report discussed in my previous entry. It appears that I misread the key piece of information:

For both the [abstinence] program and control group youth, the reported mean age at first intercourse was identical, 14.9 years. This age is seemingly young, but recall that the outcome is defined only for youth who reported having had sex and the average age of the evaluation sample was less than 17.

Getting means and medians sorted out: The key statistic in most of the countries I quoted in my previous entry is (as I should have guessed) the median date for first sexual intercourse, since it is established once the 51st percentile have lost their virginity, no matter how long it takes the other 49% of your sample.

In the case of the American survey, 49% of the sample were still virgins at the end, and we are told that the mean and modal age of all surveyed was 16, so I guess that will make the median date for first sexual intercourse of the whole sample also around 16; which is low-ish by international standards, but not quite as unusually low as 14.9.