Partial pension for less than eighteen years service—When authorized, amount

Any judge of the supreme court, court of appeals, or superior court of this state who shall leave judicial service at any time after having served as a judge of any of such courts for an aggregate of twelve years shall be eligible to a partial retirement pension in a percentage of the pension provided in this chapter as determined by the proportion his or her years of judicial service bears to eighteen and shall receive the same upon attainment of age seventy, or eighteen years after the commencement of such judicial service, whichever shall occur first.