Why does French use “avoir” to say your age?

In French you don’t be your age, you have it. Age uses avoir, never être: j’ai 30 ans is literally “I have 30 years.” Drop the ans and “j’ai trente” just means “I have thirty” — the word for “years” is obligatory. The payoff: whenever English reaches for “to be” with a number of years, French reaches for “to have.”

The full pattern conjugates avoir across persons: tu as 20 ans, il a 25 ans, quel âge as-tu ? — j’ai 30 ans. The trap is real enough that the clip warns even intermediate learners still say je suis 30 ans, copying English “I am.” Note the contrast in the cheeky reply Tu es vieux (“You’re old”): there être is correct, because vieux is an adjective, not a number of years. So the rule is precise — avoir for the age itself, être only for describing it.