Oh, yes, you are sixteen now!
Wie alt bist Du? means: how old are you?
"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 durcheinander du bist verwirrt
"Du bist faszinierend." "Du bist erstaunlich."
"Du bist alles" means "You are everything".
Du bist die Ruh' = you are the peace.
Du bist meine Liebste. - You are my beloved. Du bist meine liebste Oma. - You are my dearest grandma./You are my favourite grandma. Du bist mir die Liebste. - You are most dear to me./You are my favourite.
Du bist blöd, Du bist doof, Du bist albern,