Who aare the Na'arei B'nei Yisrael, whom Moshe sent to bring Korbanos?
Rashi: They were the firstborn. 1
Ramban #2: It refers to the youths who had never tasted the taste of sin (who had never been intimate with a woman). 2
Rosh: They are Sheluchim. We need not say that they were young. We find that "Gechazi Na'ar Elisha" (Melachim 2:5:20; he already had three mature sons) "Yehoshua Bin Nun Na'ar" (Shemos 33:11; Ibn Ezra - he was 56) and "Shnei Ne'arav" of Avraham (Bereishis 22:3; Yishmael was 13 3 ) were old.
Targum Yonason: Seeing as the Mishkan had not yet been erected and the Kohanim had not yet been chosen. Ramban (citing Targum Onkelos) - they are called Na'arim, in contrast to "the elders" (Pasuk 9), to show that they were not sent due to their Chachmah, rather, because they were firstborns.
Since they are holy and worthy of bringing sacrifices (See Berachos, 43b) [Ramban]).
Clearly the text should say '50'. Rashi says that his other Na'ar was Eliezer, who helped fight the four kings 62 years earlier (based on Tosfos Shabbos 10b DH v'Shel).
Why does the Pasuk refer to the firstborn as 'youths'?
Ramban: It does so in contrast to "the elders" mentioned in Pasuk 9.
Why did Yisrael bring specifically bulls?
In fact, this was the source of their mistake in building the Golden Calf (based on the fact that they were currently in the desert - a location of destruction and desolation [R. Bachye]) as the Ramban explains in Shemos 32:1 DH 've'Hinei Aharon' (only there, he ascribes the choice of a calf to Aharon).