Vayeishev, Breishis 37:2. Na’ar.

The passuk refers to Yosef as a "na'ar," a child. Rashi says that he was 17 years old, and the term na'ar would normally not apply to a young man of 17. He was called na’ar because he did ma’asei na’arus– he was too self absorbed, he carefully combed his hair and looked in the mirror. As the Medrash says, later, when Yosef was tempted by the wife of Potiphar, the Satan was looking for a weak spot in Yosef's personality. When the Satan saw Yosef so carefully grooming himself, he said "Hadein Didi Hu," "This one's mine."

I heard an interesting thing in the name of the Imrei Emes; (The yichus of the vort is that I heard it from Harav Yisroel Hersh Nekritz who heard it from Rav Jaeger of Shaar Yashuv.)

The Imrei Emes would pay a visit to the Rov of any town he came to. On one such visit, he told the local Rov a Rov the following vort on this passuk.

This is not the only time we find the the word "na’ar" applied to an older person. At the akeidah, Avraham told Yishmael and Eliezer "ואני והנער נלכה עד כה..." There, too, the Torah refers to the 37 year old Yitzchak as a na'ar. Why is there no discussion of teh incongruity of his being called na’ar? The answer is that there is a big difference between the Torah calling you a na’ar and your father calling you a na’ar. To a father, you’re a na’ar no matter how old you are, and that’s just the natural thing to be called.

As he was leaving the Rov’s house, an elderly almanah called out to the Imrei Emes, Rebbe! Give me a bracha! So he gave her a bracha. Then she said, "Rebbe, ah bracha fahr der kind!" (a bracha for the child.) Der Kind, her son, was in his seventies, but to her, he was a na’ar. So the Imrei Emes was able to demonstrate the truth of his vort on the spot.

Just a side comment:
Although speakers of Yiddish assume that the Yiddish word Na'ar (child/juvenile) is related to the Germen Narre (fool), this is not the case. The similarity is coincidental. Whether the Yiddish word, Naar, developed from the Hebrew or the German is utterly unknowable. It is, of course, most likely that the conceptual and linguistic resonance of the two homophones encouraged the incorporation into Yiddish. See here,  Paolo Agostini's comment.  (Here's what he says, in case of link rot:)
However, I'd like to point out that IMHO the Yiddish word nar [נאר] 'a fool' 
(e.g. haltn/shteln tsum nar, 'to make a fool of'; narn 'to deceive, to 
fool', narish 'foolish, silly, stupid, dumb', narishkayt 'folly, 
foolishness', etc.) has nothing to do with Hebrew na'ar  [נער] 'boy, lad, 
youth; servant' etc. The Yiddish word is a borrowing from German Narr 
(Middle High German: narre, Old High German narro) 'fool, crazy' (zum Narren 
halten/stellen 'to make a fool of s.o.', narren 'to fool s.o.', Narrheit 
'tomfoolery', etc.). On the other hand though, the origin of the German word 
is unsure, since it does not occur in other Teutonic languages. It is 
thought to derive from late Latin nario 'Nasenruempfer, Spoetter', i.e. 
's.o. who turns up his nase, s.o. who makes fun of s.g., mocker'. In fact, 
in OHG times the word was also used in the meaning of 'court jester, court 
fool'.