Section 66A. No person shall be employed by the metropolitan district commission as a beach or swimming pool lifeguard unless he has attained his eighteenth birthday, except that said commission may, if there is no suitable list of eligible applicants to fill existing vacancies, appoint subject to the provisions of chapter thirty-one, except for age requirements, applicants who have attained the age of sixteen years as beach or swimming pool lifeguards, provided that such applicants meet all other requirements.