Wie alt bist Du? means: how old are you?
du bist süß weil du nett bist
You are nice. = Du bist nett.
Du bist unsympathisch.orDu bist nicht nett
You are (so) sweet. - Du bist (so) süß. That's so sweet of you. - Das ist so süß von dir.
Bist du = are you (informal, singular)
"Are you okay?" or "What do you have?" is the translation into English.
It means "When will you be at home?"
translated it means are you ready
"Bist du zurück/zurueck" means "Are you back" "Have you returnded"
"Are you with me?" That would be a translation of the question "Bist du bei mir?". The start of the sentence as in the Bach/Stölzel aria "Bist du bei mir, geh' ich mit Freuden" would be better translated as "If you are with me (or: if you're near to me), I go light-hearted (or: with gladness)".
"Du bist alles" means "You are everything".