Albert Barnes Commentary Numbers 8:24

"This is that which belongeth unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service in the work of the tent of meeting:" — Numbers 8:24 (ASV)

Twenty and five years old and upward - The permanent limit as distinguished from the temporary (Numbers 4:3, Numbers 4:23, Numbers 4:30), though David found it necessary to extend the period of the Levites’ service by causing it to commence at 20 years of age (1 Chronicles 23:24–28). This rule continued in force from the time of David onward (compare 2 Chronicles 31:17; Ezra 3:8).