He is forty years old.

He is forty.

He is forty of age.

Which one (s) is/are correct?

"he is forty (40) years old"and "he is forty (40)"

He is forty years old because forty is an age, therefore you must put that it is an age, otherwise one would think you are calling him the number forty. He is forty years old. By saying he is just forty, it sounds like that is his name. He is forty of age does not make sense.