In several places, parsha vayigash refers to Benjamin as a youth (na-ar). But he must have been close to 30 years old since Joseph was at that time already in Egypt for about 20 years. Why is Benjamin referred to as a youth?
In Exodus 33:11 Joshua is referred to as a "na'ar" as well (my emphasis):
And the LORD spoke unto Moses face to face, as a man speaketh unto his friend. And he would return into the camp; but his minister Joshua, the son of Nun, a young man, departed not out of the Tent. (Mechon-Mamre translation)
Many of the commentaries there raise the issue of how he could be called a "na'ar" if he was already 57 years old at the time, and most of them answer that "na'ar" is used to refer to anyone who serves someone since such people tend to be youths. For instance, R. Abraham the son of Rambam formulates it as follows:
נקרא נער מפני שהיה משרת משה וקריאת שם זו נופלת על המשרת מפני שהוא נער על פי הרוב אע"פ שהוא ע"ה היה בעת ההיא בן שבע וחמשים שנה כמו שביאר אבא מרי ז"ל במורה
He is called "na'ar" because he was serving Moses, and the calling by this title falls on one who serves because most of the time [such a person] is a "na'ar", despite the fact that [Joshua] was at that time 57 years old as explained by my father of blessed memory in Guide [for the Perplexed].
R. Abraham is presumably referring to what Rambam wrote in Guide for the Perplexed 2:32:
As to the words, "For I am young" (ibid. ver. 6), it is well known that the pious Joseph, when he was thirty years old, is called by the Hebrew "young" (na‘ar); also Joshua, when he was nearly sixty years old. For the statement, "and his minister Joshua, the son of Nun, was young," occurs in the account of the Golden Calf (Exod. xxxiii. 11). Moses was then eighty-one years old, he lived one hundred and twenty years; Joshua, who survived him fourteen years, lived one hundred and ten years and must consequently have been at least fifty-seven years old at the time when the Golden Calf was made, and yet he is called na‘ar, "young." (Friedlander translation)
Some of the other commentaries also provide other examples in Scripture where a non-youth is referred to as "na'ar".
Accordingly, it is no surprise that Benjamin is referred to as "na'ar". In addition to being the youngest of Jacob's children, he also served Jacob and would thus earn the title "na'ar". For instance, Radak explaining the brothers' statement to Joseph that their youngest brother remained at home says:
ונשאר לשרת אבינו
He remained to serve our father.