Except as provided in subsection (c) of this Code section, a person convicted of the offense of statutory rape shall be punished by imprisonment for not less than one nor more than 20 years; provided, however, that if the person so convicted is 21 years of age or older, such person shall be punished by imprisonment for not less than ten nor more than 20 years. Any person convicted under this subsection of the offense of statutory rape shall, in addition, be subject to the sentencing and punishment provisions of Code Section 17-10-6.2.