18 U.S.C. § 2243 : US Code - Section 2243: Sexual abuse of a minor or ward

      (a) Of a Minor. - Whoever, in the special maritime and
    territorial jurisdiction of the United States or in a Federal
    prison, or in any prison, institution, or facility in which persons
    are held in custody by direction of or pursuant to a contract or
    agreement with the head of any Federal department or agency,
    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, or in any
    prison, institution, or facility in which persons are held in
    custody by direction of or pursuant to a contract or agreement with
    the head of any Federal department or agency, 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 15 years, 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.