Numbers 8:20-26 - The Levites Are Dedicated To The Lord
20 And Moses and Aaron and all the congregation of the children of Israel did unto the Levites according unto all that the LORD commanded Moses. 21 And the Levites purified themselves, and washed their clothes. And Aaron waved them before the LORD, and made an atonement for them to cleanse them. 22 And after that, they went in to do their service in the tabernacle of witness before Aaron and his sons. And according as the LORD had commanded Moses as concerning the Levites, even so they did unto them.
23 And the LORD spake unto Moses, saying, 24 "This shall be the manner of the Levites: from twenty five years upward they shall go in to wait upon the service in the tabernacle of witness, 25 and at fifty they shall cease waiting upon the service thereof, and shall labour no more: 26 but shall minister unto their brethren in the tabernacle of witness, and there wait, but shall do no more service. And see that thou do after this manner unto the Levites in their waiting times."