Numbers 8:23-26

23Now the Lord spoke to Moses, saying,

24“This is what applies to the Levites: from twenty-five years old and upward they shall enter to perform service in the work of the Tent of Meeting,

25but at the age of fifty years, they shall retire from the service of the [tabernacle] work and serve no longer.

26They may assist their brothers in the Tent of Meeting to keep an obligation, but they shall do no [heavy or difficult] work. Thus you shall deal with the Levites concerning their obligations.”