"J'ai [your age] ans."
For example, if you are 25 years old, you would say:
"J'ai 25 ans."
Explanation:
* Quel âge as-tu? means "How old are you?"
* J'ai means "I have"
* [your age] is the number representing your age.
* ans means "years"
So the full phrase translates to "I am [your age] years old."