It's not an issue of morality. It's an issue of maturity. Teenagers aren't ready to make decisions and support a child if the women were to get pregnant.
Plus, it's really not that hard to wait until you're 18. It's called will power.
It isn't hard, but what if someone doesn't want to?
What if both parties agree to have loveual intercourse, and use protection. And if the women is pregnant, and they decide to get an abortion.