21 U.S.C. § 859 : US Code - Section 859: Distribution to persons under age twenty-one

    (a) First offense
      Except as provided in section 860 of this title, any person at
    least eighteen years of age who violates section 841(a)(1) of this
    title by distributing a controlled substance to a person under
    twenty-one years of age is (except as provided in subsection (b) of
    this section) subject to (1) twice the maximum punishment
    authorized by section 841(b) of this title, and (2) at least twice
    any term of supervised release authorized by section 841(b) of this
    title, for a first offense involving the same controlled substance
    and schedule. Except to the extent a greater minimum sentence is
    otherwise provided by section 841(b) of this title, a term of
    imprisonment under this subsection shall be not less than one year.
    The mandatory minimum sentencing provisions of this subsection
    shall not apply to offenses involving 5 grams or less of marihuana.
    (b) Second offense
      Except as provided in section 860 of this title, any person at
    least eighteen years of age who violates section 841(a)(1) of this
    title by distributing a controlled substance to a person under
    twenty-one years of age after a prior conviction under subsection
    (a) of this section (or under section 333(b) of this title as in
    effect prior to May 1, 1971) has become final, is subject to (1)
    three times the maximum punishment authorized by section 841(b) of
    this title, and (2) at least three times any term of supervised
    release authorized by section 841(b) of this title, for a second or
    subsequent offense involving the same controlled substance and
    schedule. Except to the extent a greater minimum sentence is
    otherwise provided by section 841(b) of this title, a term of
    imprisonment under this subsection shall be not less than one year.
    Penalties for third and subsequent convictions shall be governed by
    section 841(b)(1)(A) of this title.