Yes there is, and it just happens to be a law.
Basing my answer of the state you reside in (Louisiana) I found a web site that shows the law in Louisiana that details age of consent: link
If you look at the description under the Felony clause, at parts A-1, and A-2, it prescribes the age range tolerance.
It appears in A-2, that the State of Louisiana considers age of consent for minors to begin at 15, if the older half of the incident is 17. Any younger than 15, and the 17 year old would be convicted of statutory rape.
In the case of an adult (over the age of 18), A-1 prescribes that the younger half would have to be at least 17. Any younger, and the adult would be convicted of statutory rape.
Based on this, it would appear that age of consent in Louisiana begins at 17 when considering adult relations. In other words, even though the federal government only considers you an adult on your 18th birthday, Louisiana believes you to be able to make adult decisions related to sexual intercourse at age 17.
Because I am not well versed in Lousiana statutes, I would suggest consulting with a legal professional such as a paralegal if this is something you need a definitive answer on.
Though I am basing my answer on what the written law of Louisiana states, there may be an amendment that was not updated at the time of this publishing, so be sure to cover your bases.
There's nothing worse than getting a loved one into trouble with the law.