A common way of asking somebody’s age is:
How old are you?
How old is your younger brother?
The question can be answered by using the verb be, such as:
I’m forty.
He’s seven.
She’s thirty years old.
Usually, we do not use the word age to describe it. However, it can be used in formal situations, for example:
Age, please. (someone filling in a form for you)
What’s your age? (a doctor asking your age more directly)
I’m forty years of age. / She’s thirty years of age. (more formal)