U.S.C. Title 18 - Crimes and Criminal Procedure Part I - Crimes Chapter 109A - Sexual Abuse.

Section 2243. Sexual Abuse of a Minor or Ward (1/5/99)

(a) Of a Minor. - Whoever, in the special maritime and territorial jurisdiction of the United States or in a Federal prison, knowingly engages in a sexual act with another person who -
(1) has attained the age of 12 years but has not attained the age of 16 years; and
(2) is at least four years younger than the person so engaging; or attempts to do so, shall be fined under this title, imprisoned not more than 15 years, or both.

(b) Of a Ward. - Whoever, in the special maritime and territorial jurisdiction of the United States or in a Federal prison, knowingly engages in a sexual act with another person who is -
(1) in official detention; and
(2) under the custodial, supervisory, or disciplinary authority of the person so engaging; or attempts to do so, shall be fined under this title, imprisoned not more than one year, or both.

(c) Defenses. -
(1) In a prosecution under subsection (a) of this section, it is a defense, which the defendant must establish by a preponderance of the evidence, that the defendant reasonably believed that the other person had attained the age of 16 years.
(2) In a prosecution under this section, it is a defense, which the defendant must establish by a preponderance of the evidence, that the persons engaging in the sexual act were at that time married to each other.

(d) State of Mind Proof Requirement. - In a prosecution under subsection (a) of this section, the Government need not prove that the defendant knew-
(1) the age of the other person engaging in the sexual act; or
(2) that the requisite age difference existed between the persons so engaging.