Numbers 8:23-26

23And the LORD said to Moses, 24“This applies to the Levites: Men twenty-five years of age or older shall enter to perform the service in the work at the Tent of Meeting. 25But at the age of fifty, they must retire from performing the work and no longer serve.

26After that, they may assist their brothers in fulfilling their duties at the Tent of Meeting, but they themselves are not to do the work. This is how you are to assign responsibilities to the Levites.”