If we talk about the age we can use two forms: a) mam 18 lat (it is more common, it says that "i have 18 year['s experiance in life]" or something like this) or b) jestem osiemnastolaktą (girl) lub jestem osiemnastolatkiem (boy) - this is longer and less common, but can be translated into "i am 18"

Just because. Why do you think that "I am 18" ist better than "I have 18" or anything else? :))))