"This chart that MineralMan cited to "prove" that Europe's attitudes about sex result in fewer teen pregnancies/abortions/STDs says that in America "Typical age of first sexual intercourse is 17.4 years". (For France and the Netherlands it is listed as 18.0 and 17.7 years respectively)."
Stats on teenage sexual activity vary widely. It's difficult to find any figures that show the age at which someone first has sex, although most agree that less than 20% have sex before 15. Another stat I've seen, though, says that 80% have had sex during their teens, which would include some married people, since the teens end at 19.
I suspect also that the stats vary from region to region within the US.
Any comprehensive study on teenage sexual behavior has to list "what" activities they are engaging in, with members of same/opposite sex, as well as listing all of the ages at which they "begin" activity (and whether they are married or not).
Anything else leaves the study too open to manipulation and misinterpretation.
If we are told that 20% have sex by age 16 and "some" are having it as young as 11 (AND drunk/passed out at the time), then there are plenty of figures that we don't know (and have to admit that at least some of the sex they engage in is NOT consensual).