Numbers 4:3,30,47
From thirty years old and upward even until fifty years old, all that enter into the host, to do the work in the tabernacle of the congregation.
Nave's Topical Index—AKJV

Numbers 8:23-26
And the LORD spoke to Moses, saying,
Nave's Topical Index—AKJV

1 Chronicles 23:3,24,27
Now the Levites were numbered from the age of thirty years and upward: and their number by their polls, man by man, was thirty and eight thousand.
Nave's Topical Index—AKJV

Ezra 3:8
Now in the second year of their coming to the house of God at Jerusalem, in the second month, began Zerubbabel the son of Shealtiel, and Jeshua the son of Jozadak, and the remnant of their brothers the priests and the Levites, and all they that were come out of the captivity to Jerusalem; and appointed the Levites, from twenty years old and upward, to set forward the work of the house of the LORD.
Nave's Topical Index—AKJV