Ok, i couldnt find much:
Statutory rape is defined as sexual intercourse with a person under a specified age. This age varies from state to state and country to country but usually ranges from 12 to 18 years. Sexual intercourse with a person who is mentally deficient or unconscious and therefore incapable of giving consent is also sometimes considered statutory rape.
From what i have read, it varies alot from state to state.
Lets say you live in Florida for example. And lets say that the law says that you can not have sex under 16. Lets say both you and your partner were 15. That is still illegal. You would probably be sent to jeuvenile court and yada yada yada. But if you keep it on the DL, nothing should happen to you (unless the girl cries foul, and then you will be hit with rape charges up the wazoo no matter the age. Then you would have to register as a sex offender. Not good.)
So legaly, it depends on the age. But who says you have to tell anyone if you are underaged.