The Japanese have sex later than Americans, but not that much later, about a third have sex by age 17. But, Japanese youths know less than Americans about sex. I'm oversimplifying, of course. Read the whole thing.
Also, as background, consider that condoms are pretty much the exclusive means of contraception short of sterilization used in Japan. The pill and IUDs are virtually non-existent there and the pill was only recently legalized there.