Section 9. For any occupation within the scope of the minimum fair wage law, the commissioner may cause to be issued to an employer of any learner, or of an employee under an approved apprentice training program, or of an employee whose earning capacity is impaired by age or physical or mental deficiency or injury, or of an employee who is certified by the secretary of health and human services or his designee as a handicapped person, a special certificate authorizing employment at such wages, less than the established minimum fair wage rates, and for such period of time, as shall be fixed by the commissioner and stated in the certificate.