נָשֹׂא, אֶת-רֹאשׁ בְּנֵי קְהָת, מִתּוֹךְ, בְּנֵי לֵוִי--לְמִשְׁפְּחֹתָם, לְבֵית אֲבֹתָם.
מִבֶּן שְׁלֹשִׁים שָׁנָה וָמַעְלָה, וְעַד בֶּן-חֲמִשִּׁים שָׁנָה--כָּל-בָּא, לַצָּבָא, לַעֲשׂוֹת מְלָאכָה, בְּאֹהֶל מוֹעֵד.
'Take the sum of the sons of Kohath from among the sons of Levi, by their families, by their fathers' houses, from thirty years old and upward even until fifty years old, all that enter upon the service, to do work in the tent of meeting. (Numbers 4:2-3).
נָשֹׂא, אֶת-רֹאשׁ בְּנֵי גֵרְשׁוֹן--גַּם-הֵם: לְבֵית אֲבֹתָם, לְמִשְׁפְּחֹתָם.
מִבֶּן שְׁלֹשִׁים שָׁנָה וָמַעְלָה, עַד בֶּן-חֲמִשִּׁים שָׁנָה--תִּפְקֹד אוֹתָם: כָּל-הַבָּא לִצְבֹא צָבָא, לַעֲבֹד עֲבֹדָה בְּאֹהֶל מוֹעֵד.
'Take the sum of the sons of Gershon also, by their fathers' houses, by their families; 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.(ibid 22-23).
בְּנֵי, מְרָרִי--לְמִשְׁפְּחֹתָם לְבֵית-אֲבֹתָם, תִּפְקֹד אֹתָם.
מִבֶּן שְׁלֹשִׁים שָׁנָה וָמַעְלָה, וְעַד בֶּן-חֲמִשִּׁים שָׁנָה--תִּפְקְדֵם: כָּל-הַבָּא, לַצָּבָא, לַעֲבֹד, אֶת-עֲבֹדַת אֹהֶל מוֹעֵד.
As for the sons of Merari, thou shalt number them by their families, by their fathers' houses; from thirty years old and upward even unto fifty years old shalt thou number them, every one that entereth upon the service, to do the work of the tent of meeting.(ibid 29-30).
While in Parshas Behaloscha it says:
זֹאת, אֲשֶׁר לַלְוִיִּם: מִבֶּן חָמֵשׁ וְעֶשְׂרִים שָׁנָה, וָמַעְלָה, יָבוֹא לִצְבֹא צָבָא, בַּעֲבֹדַת אֹהֶל מוֹעֵד.
וּמִבֶּן חֲמִשִּׁים שָׁנָה, יָשׁוּב מִצְּבָא הָעֲבֹדָה; וְלֹא יַעֲבֹד, עוֹד.
'This is that which pertaineth unto the Levites: from twenty and five years old and upward they shall go in to perform the service in the work of the tent of meeting; and from the age of fifty years they shall return from the service of the work, and shall serve no more;(ibid 8:24-25).
The Meforshim have various explanations for this discrepancy. First there is RSH"I who as usual has a somewhat Midrashic explanation (taken from the Gemarah) that the Levi would come to learn his trade at the age ot twenty five for five years, at which point he would be ready to start at the prescribed age of thirty. Nu, enough said.
The Ibn Ezra writes that the Posuk in which the prescribed age is thirty doesn't contradict the latter Posuk because at the age of thirty the Levi would start carrying the Mishkan while at the age of twenty five he only serviced it. To which the RMB"N rightfully replies that it's incorrect because in 4:23 it says כָּל-הַבָּא לִצְבֹא צָבָא, לַעֲבֹד עֲבֹדָה בְּאֹהֶל מוֹעֵד
and in the next Posuk: זֹאת עֲבֹדַת, מִשְׁפְּחֹת הַגֵּרְשֻׁנִּי--לַעֲבֹד, וּלְמַשָּׂא
so this doesn't work obviously.
The RMB"N himself offers the following explanation: Moshe was commanded to count the Leviim only from age thirty, at which point they were to be pressed into service. At age twenty five though, a Levi was able to come forth and serve of his own volition, but could not have an official post as overseer and so on. The reason being that a decadal birthday is known to a man's family and friends, while other birthdays not so much. The thirtieth birthday was thus a convenient age for counting. The RMB"N notes that in another instance, Chronicles i 23:3, the Leviim were also counted at age thirty.
I think this is the best Terutz so far, but I think it's lacking. First, the phraseology is the same in all the Psukim, indicating that the status and terms of service are the same. Second, in the Psukim where they are counted at thirty, it says: כָּל-בָּא, לַצָּבָא, or כָּל-הַבָּא לִצְבֹא צָבָא, which means that those counted included all that were to serve in the Mishkan. Another problem is obviously that the RMB"Ns differentiation between the two ages has nothing to support it from the text, other than the fact that here the Leviim are counted and here not, which may have many reasons other than his.
The usual real Terutz in such cases doesn't work here, because all of these Psukim are attributed to the same source, which all have the unmistakable imprint of P. So what to do? Did the author forget in chapter eight what he wrote in chapter 4. It's not only the age that's different, it seems that in 8, the author seems oblivious to the fact that the Leviim were just counted extensively for this purpose. Is there an obvious real Terutz I didn't grasp. Or maybe, is one of those Farkvetchte Terutzim the real thing? Did the author actually intend to have his words dissected in a semi-code manner?