The provisions of this section shall not apply with respect to any individual who is an “aged, blind, or disabled individual” for purposes of this subchapter by reason of blindness (as determined under section 1382c(a)(2) of this title ) or disability (as determined under section 1382c(a)(3) of this title ), from and after the onset of the impairment, if such blindness or disability commenced after the date of such individual’s admission into the United States for permanent residence.