Any probate judge serving on December 27, 1973, who elects to become a member of the retirement fund hereby established, and any probate judge assuming such office after October 1, 1976, may elect to be retired pursuant to this article if he or she qualifies under any one of the following:

(1) Has served as much as five years as a probate judge and has become permanently, physically or mentally unable to carry out his or her duties on a full-time basis, proof of such disability being made by certificate of three reputable physicians;

(2) Has served as much as 12 years as a probate judge and has reached or passed the age of 65 years;

(3) Has served as much as 15 years as probate judge and is not less than 62 years of age or has served as such for more than 15 years and has attained age 62, less one year for each year of service in excess of 15; provided, that such probate judge shall have attained not less than 60 years of age;

(4) Has served continuously as much as 10 years as probate judge and is not less than 70 years of age; or

(5) Has served for not less than 18 years or three full terms or a time equal to three full terms as a probate judge and is not less than 60 years of age; or

(6) Has served for not less than 24 years or four full terms or a time equal to four full terms as a probate judge, regardless of age, if upon electing to retire with less than 25 years of service, prior to his or her date of retirement he or she pays to the Secretary-Treasurer of the Judicial Retirement Fund a lump sum in an amount equal to the annual employer and employee contributions necessary to purchase one year of creditable service in the fund based on the final annual state salary earned by the judge during his or her most recent fiscal year of active judicial service.