In most states, it is illegal to have sex before your 18th birthday and it is a crime for someone over 18 to have sex with someone under that threshold age. There are additional, very severe penalties when the child is under 12 or 13. I'm talking going away to prison for a long, long time.
18 is very young for engaging in sex. A better rule is to wait until you have the ability to support both yourself and a child because the human body is capable of producing children whether the parents have the ability to take care of them or not. It only takes a few minutes to initiate the process of becoming a parent but, once that happens, you have a heavy-duty responsibility for the rest of your life. The question is whether feeling good for a few minutes is worth taking that on.