J'ai 17 ans.
Here's a breakdown:
* J'ai - This is the contraction of "je" (I) and "ai" (have).
* 17 - This is the number seventeen.
* ans - This means "years" in French.
You can also say:
* J'ai dix-sept ans. (More formal)
* Je suis âgé(e) de 17 ans. (More literal, meaning "I am aged of 17 years")
However, "J'ai 17 ans" is the most common and natural way to express your age in French.