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)) 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.