מִבֶּן֩ שְׁלֹשִׁ֨ים שָׁנָ֜ה וָמַ֗עְלָה עַ֛ד בֶּן־חֲמִשִּׁ֥ים שָׁנָ֖ה תִּפְקֹ֣ד אוֹתָ֑ם כָּל־הַבָּא֙ לִצְבֹ֣א צָבָ֔א לַעֲבֹ֥ד עֲבֹדָ֖ה בְּאֹ֥הֶל מוֹעֵֽד׃
from thirty years old and upward until fifty years old shalt thou number them: all that enter in to wait upon the service, to do service in the tent of meeting.
Read midrashic interpretation of Numbers 4:23: and classical rabbinic exegesis.