Numbers 4:3

3 afrom thirty years old up to fifty years old, all who can come on duty, to do the work in the tent of meeting.

Numbers 8:25-26

25And from the age of fifty years they shall withdraw from the duty of the service and serve no more. 26They minister
Hebrew He ministers
to their brothers in the tent of meeting cby keeping guard, but they shall do no service. Thus shall you do to the Levites in assigning their duties.”

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