Numbers 8:24

8:24 “This is what pertains to the Levites: At the age of twenty-five years and upward one may begin to join the company in the work of the tent of meeting,

Numbers 9:17-18

9:17 Whenever the cloud was taken up from the tabernacle, then after that the Israelites would begin their journey; and in whatever place the cloud settled, there the Israelites would make camp. 9:18 At the commandment of the Lord the Israelites would begin their journey, and at the commandment of the Lord they would make camp; as long as the cloud remained settled over the tabernacle they would camp.

Numbers 10:6

10:6 And when you blow an alarm the second time, then the camps that are located on the south side must begin to travel. An alarm must be sounded for their journeys.