Any firefighter employed in a fire department on and before the first day of January, 1947, hereinafter in this section and RCW
41.16.090 to 41.16.190 inclusive, referred to as "firefighter," and who shall have served twenty-five or more years and having attained the age of fifty-five years, as a member of the fire department, shall be eligible for retirement and shall be retired by the board upon his or her written request. Upon his or her retirement any firefighter shall be paid a pension based upon the average monthly salary drawn for the five calendar years before retirement, the number of years of his or her service and a percentage factor based upon his or her age on entering service, as follows:
Said monthly pension shall be in the amount of his or her average monthly salary for the five calendar years before retirement, times the number of years of service, times the applicable percentage factor.