How do you say she is 18 in french?

There are a couple of ways to say "she is 18" in French, depending on the context and formality:

Formal:

* Elle a dix-huit ans. (This is the most common and neutral way to say it.)

Informal:

* Elle a 18 ans. (This is a more casual way to say it, often used with friends or family.)

Note: You can also use "elle est âgée de dix-huit ans" (she is aged eighteen years) but this is less common and sounds a bit more formal.