Why is "Bist du einen Junge oder einen Mann?" not correct?
"einen" is used with accusative case. In the above question, there is no action being done on "boy" or "man" by "you" (nominative). "You" simply is either "boy" or "man", which makes them nominative as well. And "ein" is the nominative form.
Why not "Bist du ein Junge, sondern ein Mann"?
sondern=rather. I think, it may be "You are not a boy, rather a man" translated with "sondern", but English sentence is other.
Yet not, Sind sie ein Junge oder ein Mann
NB: polite form needs to be capitalised: Sie
Why not Seid ihr ein Junge oder ein Mann?
I think because it refers to a single person. While ihr is used to address multiple people.
I thought Junge means young i still don't understand
Der Junge is a boy, jung ist young, I think
And a girl in german is mädchen
No, that's not quite correct. The phrase 'a girl' is "eine Mädchen" in Geman. Remember, capitalization COUNTS!!