Numbers 8:26

but shall minister with their brethren in the Tabernacle of the Congregation, to keep the charge, have the supervision over the furniture and the vessels of the Sanctuary, and shall do no service. Thus shalt thou do unto the Levites touching their charge, with regard to all the services which they were to perform. It was not a complete retirement which the Levites suffered, but they were relieved of the more difficult manual and physical labor connected with the offering of the sacrifices and with the service of the Tabernacle. David later modified these regulations by having all the Levites serve from the time they were twenty years old, 1 Chronicles 23:24, and this ordinance remained in power while the kingdom of Judah existed.