Formal:
* J'ai treize ans. (pronounced "zhay trez ahns") - This is the most formal and literal translation.
Informal:
* J'ai 13 ans. (pronounced "zhay trez ahns") - This is a more casual way of saying it.
* J'ai treize piges. (pronounced "zhay trez peezh") - This is a more colloquial way of saying "I am 13 years old" and is often used among friends or in informal situations.
Note: In French, the number "13" is written as "treize" and pronounced "trez".