Numbers 8:23-26 - Retirement Of The Levites
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."