Numbers 8:23-26

And the Lord said to Moses, This is the rule for the Levites: those of twenty-five years old and over are to go in and do the work of the Tent of meeting; But after they are fifty years old, they are to give up their work and do no more; But be with their brothers in the Tent of meeting, taking care of it but doing no work. This is what you are to do in connection with the Levites and their work.