The Secretary, to the extent necessary in order to prevent curtailment of opportunities for employment, shall by regulations or by orders provide for the employment of learners, of apprentices, and of messengers employed primarily in delivering letters and messages, under special certificates issued pursuant to regulations of the Secretary, at such wages lower than the minimum wage applicable under section
206 of this title and subject to such limitations as to time, number, proportion, and length of service as the Secretary shall prescribe.