ANDY'S ANSWER: Interestingly enough, I just saw a data map that indicated that the average age at which North American teens become sexually active is 18.1 for Canada, 18.0 for the US and 18.7 for Mexico. The average age for girls is slightly lower than the average age for boys.
The country with the lowest average age is Iceland (15.6) and Malaysia has the oldest average age with 23.0.