I wouldn’t call it a sexual maturity because early sexual contacts don’t mean that person has grown to be mature. Sexual relationships can be started by not grown up teenagers too. It’s connected with many social, economical and global factors. Often we can call it a difference between old and young generations. The tolerance towards sex in whole in society has noticeably increased lately. Also has increased tolerance towards different expressions of sex, including homosexuality – if 20 years ago nobody even talked about it, then today you can talk about it freely.
One more factor is re-sexualization of woman. That means that woman has become more sexually active. She can ask her man to a date, go to movies or even propose to him.