כִּ֣י בְדִבְרֵ֤י דָוִיד֙ הָאַ֣חֲרֹנִ֔ים הֵ֖מָּה מִסְפַּ֣ר בְּנֵי־לֵוִ֑י מִבֶּ֛ן עֶשְׂרִ֥ים שָׁנָ֖ה וּלְמָֽעְלָה׃
For by the last ordinances of David the sons of Levi were numbered from twenty years old and upward.
Read midrashic interpretation of I Chronicles 23:27: and classical rabbinic exegesis.