(a) Any district judge shall be eligible for retirement and may elect to be retired pursuant to this article if he:

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

(2) Has served for 12 years as a district judge or judge of a county court and has reached or passed the age of 65 years;

(3) Has served for 15 years as a district judge or judge of a county court 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, however, that no district court judge shall retire under the provision of this subdivision prior to attaining the age of 60 years;

(4) Has served for 10 years as a district judge or judge of a county court 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 circuit judge, district judge, judge of a county court or any two or more of the positions; provided, that he shall not be less than 60 years of age.

(b) Any provision of this article to the contrary notwithstanding, a district court judge shall not be eligible to retire on service or otherwise receive service retirement benefits on account of his participation in said fund, unless such member shall have attained 60 years of age; provided, however, that nothing in this subsection shall be construed as limiting, altering or amending existing provisions of law relating to eligibility for entitlement to disability benefits.

(c) Any former judge of a county court, former district attorney or former deputy district attorney serving as circuit judge on January 16, 1977, shall have the right, within one year from January 16, 1977, to elect to come under the provisions of Article 1 of this chapter, receiving prior service credit under such article for years served as a judge of a county court, district attorney, or deputy district attorney to the same extent that years served as a circuit judge would render the circuit judge eligible for retirement, notwithstanding any contrary provisions of Article 1 of this chapter. The notice of election to come within Article 1 of this chapter shall be filed with the Clerk of the Supreme Court.