‘3. Proof of age. The superintendent may issue sign a permit only upon receiving and examining satisfactory evidence of the minor's age. Satisfactory evidence consists of a certified copy of the minor's birth certificate or baptismal record, a passport showing the date of birth or other documentary evidence of age satisfactory to the superintendent and approved by the director. The superintendent may require, in doubtful cases, a certificate signed by a physician appointed by the school board, stating that the minor has been examined and, in that physician's opinion, has reached the normal development of a minor of the same age and is in sufficiently sound health and physically able to perform the work the minor intends to do.’