How do you say how in French meaning old?

There are several ways to say "how old" in French, depending on the context and formality:

Formal:

* Quel âge avez-vous? (What is your age?)

* Quel âge a-t-il/elle? (What is his/her age?)

Informal:

* Tu as quel âge? (How old are you?)

* Il/Elle a quel âge? (How old is he/she?)

Other options:

* Combien d'années as-tu? (How many years old are you?)

* Il/Elle a combien d'années? (How many years old is he/she?)

The best option will depend on your relationship with the person you are asking and the level of formality of the situation.