quel âge as-tu? . how does this sentence mean How old are you when as tu means do you have?How did this sentence formation took place?
Quel âge a-tu? means what is already posted like: what age do you have?
How was it formed let's translate it.
Quel = What
âge = age
as = verbe avoir which is to have.
tu = you
Try making a sentence from these words, you'll get "What age do you have?"
It is also equivalent to what is your age? but that could be translated to quel est ton(votre) âge? since is is verb to be which is the English counterpart of être.