From the U.S. Code Online via GPO Access
[wais.access.gpo.gov]
[Laws in effect as of January 3, 2006]
[CITE: 18USC2243]
TITLE 18--CRIMES AND CRIMINAL PROCEDURE
PART I--CRIMES
CHAPTER 109A--SEXUAL ABUSE
Sec. 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 Attorney
General \1\ knowingly engages in a sexual act with another person who--
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\1\ So in original. Probably should be followed by a comma.
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(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 Attorney
General \1\ 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 five 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.