The aggregate term of service of any participant need not be continuous nor need it be confined to a single fire department or law enforcement agency nor a single municipality in this state to entitle such participant to a retirement pension if the participant has been duly enrolled in a fire department or law enforcement agency of a municipality which has elected to extend the retirement pension provisions of this chapter to its firefighters or reserve officers at the time he or she becomes eligible for the retirement pension and has paid all fees prescribed. To be eligible to the full pension a participant must have an aggregate of twenty-five years service, have made twenty-five annual payments into the fund, and be sixty-five years of age at the time the participant commences drawing the pension provided for by this chapter, all of which twenty-five years service must have been in the fire department or law enforcement agency of a municipality or municipalities which have elected to extend the retirement pension provisions of this chapter to its firefighters or reserve officers. Nothing in this chapter shall require any participant having twenty-five years active service to continue as a firefighter or reserve officer and no participant who has completed twenty-five years of active service for which annual retirement pension fees have been paid and who continues as a firefighter or reserve officer shall be required to pay any additional annual pension fees.