can't I say " they were young" ?
No, "They were young." = "Sie waren jung."
der Junge = the boy
der junge X = the young X (X = masculine noun)
The wonders of modern medicine
sie sind jetzt Mädchen :)
It depends on the context. If you want to excuse them for being young in German we would say: "Sie waren (noch) Kinder" meaning that they were young and didn't know ass from elbow.