Numbers 8:23 MSG

[23] God spoke to Moses: “These are your instructions regarding the Levites: At the age of twenty-five they will join the workforce in the Tent of Meeting; at the age of fifty they must retire from the work. They can assist their brothers in the tasks in the Tent of Meeting, but they are not permitted to do the actual work themselves. These are the ground rules for the work of the Levites.”