57.994.  First payment of benefits under system available, when — vesting of rights. — Annuity payments to retired employees under the provisions of sections 57.949 to 57.997 shall be available beginning January first next succeeding the expiration of two calendar years from the effective date of the establishment of the system to eligible retired employees, and employees with at least eight years of creditable service shall have vested rights and upon reaching the required age shall be entitled to retirement benefits.