Numbers 8:24-26

24“This is for the Levites. Men 25 years old and upward are to present themselves for service to work in the Tent of Meeting.

25But at the age of 50 he is to retire from his service and work no longer.

26He may assist his brothers in the Tent of Meeting to do their duties, but he himself will no longer do the work. Thus, you are to assign to the Levites their responsibilities.”