וְדִבֶּ֨ר יְהֹוָ֤ה אֶל־מֹשֶׁה֙ פָּנִ֣ים אֶל־פָּנִ֔ים כַּאֲשֶׁ֛ר יְדַבֵּ֥ר אִ֖ישׁ אֶל־רֵעֵ֑הוּ וְשָׁב֙ אֶל־הַֽמַּחֲנֶ֔ה וּמְשָׁ֨רְת֜וֹ יְהוֹשֻׁ֤עַ בִּן־נוּן֙ נַ֔עַר לֹ֥א יָמִ֖ישׁ מִתּ֥וֹךְ הָאֹֽהֶל׃
GOD would speak to Moses face to face, as one person speaks to another. And he would then return to the camp; but his attendant, Joshua son of Nun, [serving as] deputy,* would not stir out of the Tent.
*[serving as] deputy Or “a youth.”
(The above rendering and its footnote come from the RJPS translation, an adaptation of the NJPS translation.)
This is the only occasion where Joshua is called a נַעַר (cf. Num. 11:28). It is in the context of his serving as a subordinate and carrying out a delegated task on Moses’ behalf. This status and function are appropriate to the term’s basic sense. The personal noun נַעַר often discloses not only gender (when used in specific reference) but also social status.
In a monograph, Carolyn Leeb finds that נַעַר generally denotes not an age grade but rather a social status of being outside the normal protection offered by one’s בֵּית אָב, the corporate household that formed the basis of a person’s identity (Away from the Father’s House, 2000). She refines the conclusions reached both by Hans-Peter Stähli’s 1978 dissertation—reflected in the HALOT dictionary—that the two semantic domains for נַעַר are legal or social states [unmarried dependent male; servant], and by Lawrence Stager’s 1985 article, “The Archaeology of the Family in Ancient Israel,” which held that the נַעַר was a firstborn male who had not yet inherited his family’s estate, or a younger son who was unlikely to become the head of an autonomous household. It is often counterposed with זָקֵן, which is well known as a status term (“elder”).
In short, in the absence of contextual emphasis on age, the noun נַעַר probably evokes a relationship of dependency and subordination. (This was the rule of thumb followed by the revising translator of the Torah.) That being said, an age-related meaning cannot be ruled out.
In his comment here, Ibn Ezra cites a talmudic question as to how Joshua could have been called a “youth” when (as established by counting backwards from his age at death) he was 56 years old at the time; Ibn Ezra then explains that it’s an elliptical expression: Joshua was performing the type of service that a youth would typically perform.
Along those lines, Ramban (citing six other examples) offers a more general explanation that seems persuasive: “In my opinion, it is Hebrew’s preferred way to refer to every attendant as נַעַר: the esteemed officeholder is [called] הָאִישׁ, and the one who attends him is called נַעַר.” (This schema is roughly correct. As Ramban notes, the biblical use of אִישׁ is not based on the referent’s age or adult status. More precisely, however, it is based on his essential participation in the situation under discussion, rather than his social role.) At any rate, Joshua is called נַעַר here because he is serving as an ad hoc functionary.
The real question is what does נַעַר add here to what the previous term מְשָׁרֵת (“attendant”) has already told us about Joshua? The point seems to be that whereas a מְשָׁרֵת would normally remain physically close to the person being waited on, here Joshua remains behind whenever Moses leaves, as his stand-in. This sense of נַעַר is thus “deputy, proxy.”
As for translation into English, the NJPS rendering “but his attendant, Joshua son of Nun, a youth” reflects only the age sense of the noun in question. The revised rendering provides a more functional nuance, while preserving the NJPS construal in a new footnote.