Section 71. Such badge shall not be issued until the officer issuing the same shall have received, examined, approved and filed evidence that such boy or girl is twelve or over, which shall consist of either a birth certificate, a baptismal certificate showing the date of birth, or a certified copy of the official school record of the applicant showing his date of birth. Such officer may refuse to issue such badge to any boy or girl who in his opinion, after due investigation, is found physically or mentally incompetent or unable to do such work in addition to the regular school attendance required by law.