A paid firefighter, irrespective of date of hire, who has at least five (5) years of continuous service as defined in section 72-1403, subsections (E) and (H), Idaho Code, and who discontinues service with the city or fire district prior to meeting voluntary retirement or disability requirements, and who has not withdrawn his contributions as provided in section 72-1445 [72-1444], Idaho Code, shall be eligible, only after reaching sixty (60) years of age, to receive a monthly service retirement benefit equal to two percent (2%) of his average monthly salary, as defined in section 72-1431, Idaho Code, for each year of credited service, adjusted by the cost of living adjustment as provided under section 72-1471, Idaho Code.