Ages of Levite Workers


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A Three Way Contradiction

Bemidbar 4 describes the census of the Levites who were to serve in the Mishkan,  repeating 7 distinct times that those aged 30-50 will gather for work:

מִבֶּן שְׁלֹשִׁים שָׁנָה וָמַעְלָה וְעַד בֶּן חֲמִשִּׁים שָׁנָה כׇּל הַבָּא לַעֲבֹד עֲבֹדַת עֲבֹדָה וַעֲבֹדַת מַשָּׂא בְּאֹהֶל מוֹעֵד.
from thirty years old and upward even to fifty years old, everyone who entered in to do the work of service, and the work of bearing burdens in the Tent of Meeting,

Somewhat surprisingly, just a few chapters later, in the context of the description of the consecration of the Levites, a different age range is presented:

(כד) זֹאת אֲשֶׁר לַלְוִיִּם מִבֶּן חָמֵשׁ וְעֶשְׂרִים שָׁנָה וָמַעְלָה יָבוֹא לִצְבֹא צָבָא בַּעֲבֹדַת אֹהֶל מוֹעֵד. (כה) וּמִבֶּן חֲמִשִּׁים שָׁנָה יָשׁוּב מִצְּבָא הָעֲבֹדָה וְלֹא יַעֲבֹד עוֹד. (כו) וְשֵׁרֵת אֶת אֶחָיו בְּאֹהֶל מוֹעֵד לִשְׁמֹר מִשְׁמֶרֶת וַעֲבֹדָה לֹא יַעֲבֹד כָּכָה תַּעֲשֶׂה לַלְוִיִּם בְּמִשְׁמְרֹתָם.
(24) “This is that which belongs to the Levites: from twenty-five years old and upward they shall go in to wait on the service in the work of the Tent of Meeting; (25) and from the age of fifty years they shall cease waiting on the work, and shall serve no more, (26) but shall minister with their brothers in the Tent of Meeting, to perform the duty, and shall do no service. You shall do thus to the Levites concerning their duties.”

These verses set not 30 as the lower age limit, but 25.1 How is the seeming contradiction to be understood?  Do the two chapters speak of different circumstances? 

To make matters more confusing, in the era of David (Divrei HaYamim I 23:24-28), Chizkiyahu (Divrei HaYamim II 31:17) and Ezra (Ezra 3:8-11), it appears that the Levites began working already at the age of twenty!2  Did the law change from the period of the Wilderness and Tabernacle to the period of the Mikdash?  If so, what made the change necessary, who initiated it, and by what right?

Additional Questions

The terms used to describe the various functions of the Levites are somewhat ambiguous in meaning. Varying explanations of each might shed light on the questions above.