"We are not boys, but men."
Translation:Mi nem fiúk vagyunk, hanem férfiak.
No, you're right, we don't have to say "mi" at all. I just thought if you wanted to emphasize the fact that these fellows are not boys any longer we may want to put out "mi" to show who it refers to. Sometimes we do that in Hungarian, same in Italian, etc. The sentence without "mi" should be accepted.