Rabbi Shimon explains that the Levites represent the Left column of Israel and when they come to serve in the Tabernacle they sacrificed two bulls.
Ezekiel 1:10 “וּפְנֵי-שׁוֹר מֵהַשְּׂמֹאול” “on the left the face of an ox”
The Levites require two bulls (Left) to purify them before they start working in the Holy Tabernacle.
The priests represent the Right of Israel and the force that the body depends on. The priest had the power to correct the world but the Right needs the Left to connect to the body that is the aspect of Israel and Central column. That makes the body the main part that unifies the Right and the Left to create completion.
“זֹאת אֲשֶׁר לַלְוִיִּם מִבֶּן חָמֵשׁ וְעֶשְׂרִים שָׁנָה וָמַעְלָה יָבוֹא לִצְבֹא צָבָא בַּעֲבֹדַת אֹהֶל מוֹעֵד”
“This is what applies to the Levites: from twenty-five years old and upward they shall enter to perform service in the work of the tent of meeting.”
The Levites worked in the Holy from age 25 to 50. Rabbi Shimon explains that when A Levite comes to the 50th level, his ‘fire’ gets cold and the Light of Chokmah that comes from the Left can’t control the judgments of the Left. It’s not about the physical strength that changes with age but the transformation of the vessel.
Daily Zohar # 2748 – Behaalotcha – Why the Levites retire at age 50