An employer shall not be deemed to have knowingly employed an underage minor in the performance of contracts subject to the Act if, during the period of the employment of such minor, the employer has on file an unexpired certificate of age issued and held pursuant to regulations issued by the Secretary of Labor under section 3(1) of the Fair Labor Standards Act of 1938 (29 CFR 570.121), showing that such minor is at least 16 years of age.