29 going on 30? Whence does this expression originate?

From where does the phrase “29 going on 30” originate? For example, when someone asks “How old are you?” and the respondent answers, “I’m 18 going on 19,” as if there is any alternative. You can’t be 18 going on 20 (unless we’re dealing with leap years and sarcasm). Maybe my question is not one of origins, but one of, “why do people say this?”