If an individual is entitled to a disability insurance benefit for a month after a month for which such individual was entitled to an old-age insurance benefit, such disability insurance benefit for each month shall be reduced by the amount such old-age insurance benefit would be reduced under paragraphs (1) and (4) for such month had such individual attained retirement age (as defined in section 416( l ) of this title) in the first month for which he most recently became entitled to a disability insurance benefit.