horse terms in French and English

In my 10th year of French studies, I took a year-long course in translation. Whenever a student would ask, "but why do they say `I have 18 years' instead of `I am 18 years old?'" the teacher's answer was the same: Because French is not English using different words. In other words, just accept it. This lesson was brought home to me when I was at the movies in Paris one day watching an American film. The actress said in English, "I'm completely lost here," and the translation in French...