We talk about people’s ages with [ be + number ]
– He is thirty-five.
– She will be twenty-one next year.

or [ be + number + years old ]

– He is thirty-five years old.

To ask about somebody’s age, say How old are you? (What is your age? is correct but not usual.)

Note the structure [ be + …age ] (without preposition).

– When I was your age, I was already working.
– The two boys are the same age.
– She’s the same age as me.