I worked the movie THE BLUE AND THE GRAY as a wrangler and cavalry fight scenes. That was an exact line from the movie. They probably got the line also from the book The Common Soldier. And as the old saying goes "If the shoe fits wear it", and it don't hurt to have your age as 18 in it, if your trying to get in the Confederate Army at age 15. Who knows, but it does make a good story and a good movie line.