Quick answer
¿Cuántos años tienes?
the everyday version
¿Cuántos años tiene?
the alternative — see below for when
Which one to use
The verb is tener, not ser. The question is literally “how many years do you have?”, and the answer follows the same logic: tengo treinta años, “I have thirty years”.
So soy treinta is wrong, and so is estoy treinta. Use ¿cuántos años tienes? with friends and children, and the usted form ¿cuántos años tiene? with strangers and in official settings. The word años is normally kept in the answer.
All the versions
¿Cuántos años tienes?How old are you? — informal.
¿Cuántos años tiene?How old are you? — formal.
Tengo veintiocho años.I’m twenty-eight.
¿Cuántos años tiene tu hermano?How old is your brother?
Acabo de cumplir treinta.I’ve just turned thirty.
Where you are matters
The structure is identical everywhere; in Argentina and Uruguay the informal question is <b>¿cuántos años tenés?</b>.
Check yourself
Which sentence means “I’m thirty years old”?