מִבֶּ֨ן שְׁלֹשִׁ֤ים שָׁנָה֙ וָמַ֔עְלָה וְעַ֖ד בֶּן־חֲמִשִּׁ֣ים שָׁנָ֑ה כָּל־הַבָּא֙ לַצָּבָ֔א לַעֲבֹדָ֖ה בְּאֹ֥הֶל מוֹעֵֽד׃
from thirty years old and upward even unto fifty years old, every one that entered upon the service, for service in the tent of meeting,
Read kabbalistic (mystical) interpretation of Numbers 4:43: and classical mystical sources.