for daver re: underage sex - stats on teen sex
http://www.agi-usa.org/pubs/journals/2602100.html
This links to a research paper looking at the initiation age of intercourse for boys and girls in 14 countries (incl UK and USA but not Australia) starting at age 15. Even at this age the proportion is WAY under 50%. There are nice graphs and tables with the relevant numbers. Survey done in 2000.
