Section 20. The legislature shall prescribe by law a plan of retirement for justices and judges of courts of record, including the basis and amount of retirement pay, and requiring except as provided in section 35 of this article, that justices and judges of courts of record be retired upon reaching the age of seventy. Any retired justice or judge of any court of record who is drawing retirement pay may serve as a justice or judge of any court. When serving outside his county of residence, any such retired justice or judge shall receive his necessary traveling and subsistence expenses. A retired judge who is temporarily called back to the active duties of a judge is entitled to receive the same compensation and expenses as other like active judges less any amount received for such period in retirement benefits.