One million teenage girls get pregnant every year, in the United States. The younger you are when have sex, the more partners you are likely to have. There are many reason to wait to have sex. Religion is one. Many people wait to have sex because of their religion.
Some religions consider it a sin to have sex before marriage.
Another reason would be that if you have sex when you are younger, you and the person you have sex with, will have this tie, something like a bond that is not easily broken. When you have sex while going out with someone, the risk of being used is a definite factor. The guys might be using the girl just to get some or to just be able to brag to his friends. And vise versa. If you are going to have sex you must be ready to accept the consequences.