No minor under sixteen years of age shall be employed or permitted to work, when school is not in session, in any agricultural occupation unless the employer of such minor has received a birth certificate, an agricultural work permit issued by the Department of Education or other legal proof of age. Each employer shall retain in his possession, and make available to the commissioner or his agent for inspection, each such legal proof of age, until the termination of the employment of the minor therein named. At the termination of employment, the employer shall return to each minor upon request such legal proof of age.