II Chronicles 31:17 Midrash

וְאֵ֨ת הִתְיַחֵ֤שׂ הַכֹּהֲנִים֙ לְבֵ֣ית אֲבוֹתֵיהֶ֔ם וְהַ֨לְוִיִּ֔ם מִבֶּ֛ן עֶשְׂרִ֥ים שָׁנָ֖ה וּלְמָ֑עְלָה בְּמִשְׁמְרוֹתֵיהֶ֖ם בְּמַחְלְקוֹתֵיהֶֽם׃

and them that were reckoned by genealogy of the priests by their fathers’houses, and the Levites from twenty years old and upward, in their charges by their courses;