Man, just take a look at the ample supply of scriptural references backing up their claim !
• In Numbers 8:25, 26, concerning the Levites who served at the house of worship of Jehovah, we read: “But after the age of fifty years he will retire from the service company and serve no longer. And he must minister to his brothers in the tent of meeting in taking care of the obligation, but he must render no service. In accord with this you will do to the Levites in their obligations.” Since the appointed ministers among Jehovah’s witnesses on earth today do not retire from their service at any age, how applicable at the present time is the principle of this considerate law of Jehovah here quoted?
In the ancient nation of Israel there were three main families of Levites. All the male members of these who qualified served as assistants to the priests of Jehovah at his holy tabernacle or temple. In the course of time, as Jehovah himself foresaw, these Levite families became very populous in male members who were eligible for service at his temple sanctuary. The number of service openings for them at the temple sanctuary was limited. In order to provide for all the Levites to have a hand at enjoying these service privileges it finally became necessary for King David to divide up all the Levites in twenty-four courses, each course or division being assigned to one week of service at the temple each half year, or a total of two weeks in a full year, besides all Levites together serving at the special festivals, three times annually, in Jerusalem. It was doubtless not merely out of consideration for their age but to prevent overcrowding of such offices that Jehovah God instructed in the law stated above that those reaching the end of their fiftieth year of age should be retired from obligatory service. They could assist voluntarily those who were still eligible to serve at the temple, but they themselves were given no direct assignment nor were they held accountable to fill such. Although retired from assigned service they still received the benefit of the sacrificial offerings made by the Israelites and also of the tithes that the twelve tribes of Israel paid in support of the temple service and worship.
This does not establish any rule for Jehovah’s appointed ministers at his spiritual temple today to be retired from preaching or from any other ministerial service. The spiritual Israelites or Christians, who are anointed with God’s holy spirit for his service, are not under the Mosaic law that anciently applied to the nation of natural Israel. Since the day of Christ the Mediator, spiritual Israelites are under a new covenant with Jehovah God.
Now as regards the priests of Israel, these were not retired at the end of their fiftieth year, and the high priest himself served in his holy office until death if he continued capable. The fact is, the first high priest of Israel, Aaron the brother of Moses, was chosen for service at eighty-three years of age, and he served for almost forty years afterward. At the beginning of the Christian era the spiritual Israelite, the apostle John, was nearing a hundred years of age when he wrote five inspired books of the Holy Bible and completed the canon of the Holy Scriptures.
For the Christian there is no retiring of him from Jehovah’s service at fifty years of age, no more than there is a withdrawal of his anointing with the holy spirit of God to preach God’s Word when he reaches fifty years of age. Today the field is the world, the entire globe; and this good news of God’s established kingdom must be preached in all the inhabited earth for a witness to all the nations before the end of this old world comes. By virtue of their dedication to Jehovah God to do his will forever, Jehovah’s witnesses today are under obligation to continue in the fulfillment of their commission to preach the good news of God’s kingdom until the witness work is finished. There is no Scriptural precedent for retiring them from service, even after they have served for fifty years, and then putting them on a pension. If old age or advancing years incapacitate a dedicated Christian for a particular kind of responsibility, then he may be shifted to another form of service that he can perform. The obligations that his dedication to God imposes upon him do not allow for him to be relieved of all service responsibilities and privileges and to be retired thereafter to live at ease on a pension, to become a sheer financial burden upon a congregation or upon the organization that supervises the New World society of Jehovah’s witnesses.
The Watchtower 1958, May 15, page 319