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.