"Bist" is used only with "du",
"sind" is used with "Sie" (the one that always starts with capital letter).
You can say "Du bist jung" — "You (my son) are young"
or:"Sie sind jung" — "You (your highness, crown prince of Denmark) are young".
It is also possible to say "Ihr seid jung" — "You (children of my neighbor) are young"
In all these cases the first two words need to "play together"
In addition to Ruslan's lovely input, "sind" could also go with "wir", as in: "wir sind"(1st person plural) to mean "we are".
No, sie is they, Sie is you (formal) and sie is she.
Ist sie jung? - Ja, sie ist jung.
Is she young? Yes, she is young.
Sind sie jung? - Ja, sie sind jung.
Are they young? Yes, they are young.
Sind Sie jung? - Ja, ich bin jung.
Are you young? Yes, I am young.
Jung means young, as in age. Junge means boy. I'm not sure if this was the answer you were looking for but I thought that I should answer it this way. Just in case.
When I hear Du bist junge, it sounds like Bu bist junge. Am I hearing it right?
It should sound like "Du bist jung".
du - /du/
bist - /bɪst/
jung - /jʊŋ/
Also note that junge is the inflected form (as in der junge Mann). In a sentence like this with a predicate adjective, don't inflect the adjective.