Because of how fast a horse matures physically, weights are necessary to allow younger, weaker horses to compete against their slightly older, stronger and faster counterparts. When a horse reaches two years of age, it has reached about 95 percent of its peak growth and strength. But it will be fully mature physically when it is at the end of its second year, which means horses that are closer to three years of age generally are stronger and faster than horses that are six to 12 months younger. Because age plays such a large factor in how well younger horses run, thoroughbred horse races will use a mostly standardized “Weight for Age” scale. The scale was created in England during the 1860s and takes into account a horse’s exact age, gender, race distance and the month of the year when determining how much of an impost to place on horses. The longer the race distance, the more of a natural handicap younger horses have due to their younger, less developed bodies.