You are useless.
You are in my thoughts
Wie alt bist Du? means: how old are you?
It could be the beginning of a sentence concerning a female person. du bist eine ... - you are a ... It can also be an equivalent of the following expressions, said in resignation or exasperation to a female person, meaning: Du bist mir eine feine Freundin! - A fine friend you are! Na, du bist mir eine Expertin! - Some expert you are! It can also be said in admiration, meaning the exact opposite.
"du bist doof" or "du bist blöd" or "du bist dämlich" are equivalent to "you are stupid"
Bist du = are you (informal, singular)
Du bist eine hübsche Frau, du bist süß. = You're a pretty woman, you're sweet.
bist du in Ordnung = are you okay
du bist scharf = you are sharp
"Du bist faszinierend." "Du bist erstaunlich."
du bist durcheinander du bist verwirrt
Not sure what the meaning may be but that translates to "You are Audi".
"Du bist alles" means "You are everything".