The Director may provide by regulation for the employment in any occupation of individuals whose earning capacity is impaired by age, or physical or mental deficiency or injury at such wages lower than the minimum wage rate provided in Section 4, subsection (a), as he may find appropriate to prevent curtailment of opportunities for employment, to avoid undue hardship, and to safeguard the minimum wage rate of this Act, except that no person who maintains a production level within the limits required of other employees may be paid at less than the minimum wage. No employee shall be employed at wages fixed pursuant to this Section except under a special license issued under applicable regulations of the Director.