How do you say i am 13 years old n french?

There are a couple of ways to say "I am 13 years old" in French, depending on the level of formality:

Formal:

* J'ai treize ans. (pronounced "zhay trez ahns") - This is the most common and formal way to say it.

Informal:

* J'ai 13 ans. (pronounced "zhay trez ahns") - This is a more casual way to say it, omitting the word "treize" (thirteen). You might use this with friends or family.

Both translations are grammatically correct and understood. Choose the one that best suits the context and your relationship with the person you are talking to.