Any member of the Fire Department who, when joining, is within the ages of 18 years and 40 years, to complete the time required by law to become exempt, shall serve seven years as an active fire fighter under municipal control and will therefore be entitled to apply for an exempt certificate, which certificate of exemption shall be issued by the Borough Council upon certification by the Secretary of the fire company of which the applicant is a member and the Secretary of the Hopatcong Relief Association. Said member, upon completing the term of service of seven years, or a member of the Department incapacitated by illness and who can no longer serve as an active fire fighter, may retire as an active member upon certification by a practicing physician that said member is physically or mentally unfit for further service as an active fire fighter.