We usually just give the number when talking about ourselves or other people. We generally add on "years old" when talking about animals, plants and things in general like cars, buildings, objects, etc. Hope that helps. :)

i think, A : I'm 30 years old

B. The listener can observe that you're referring to years, not months or days.

'i'm 30' is enough when you talk with someone from my point of view.the second one is too formal.

B. I'm 30.