A U.S. federal law (U.S. Code 18, Section 2421), passed in 1910, that prohibits the transport of "any individual under the age of 18 years in interstate or foreign commerce, or in any Territory or Possession of the U.S., with intent that such individual engage in prostitution, or in a sexual activity for which any person can be charged with a criminal offense..."
The primary application of the Mann Act is against child prostitution by affording equal protection to teens and children. This overrides any state laws pertaining to the age of consent and statutory rape. However, it also applies to males or females taking an under-age partner to a hotel across the state line. The maximum term is five years in a federal prison.