I agree with everyone who's mentioned sex ed really being the best way to stave the activity. I'm not so sure about sexual morals really changing all that much in Japan though - rather, the strength and pervasiveness of the sexualized media is swaying younger kids to become sexually active. The Japanese, insofar as I know, were always pragmatic and never prudish when it came to nudity or sex (since Christianity and the Bible haven't really shaped their culture or history).

As for sex ed lacking, it certainly (still) is in Ireland - and it was really nonexistent in the late 70's, when my husband attended high school. Abortion is, however, illegal here, so they haven't exact numbers (but I'd say the percentages would be akin to that in Japan, and although birth control is available, it's too expensive for the masses to afford, besides which it's strongly discouraged by the Church).