Quel age as-tu? would more translate to What is your age? or more literally What age do you have ? to make it easy for a beginner.

It could translate to How old are you? in the meaning, but it’s a harder for a beginner.

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.