The horses that compete in the Kentucky Derby are all three years old. This is a strict age requirement for the race, designed to ensure that the horses have reached an appropriate stage of physical and mental maturity to handle the demands of the race, which is 1ΒΌ miles long
. In Thoroughbred racing, all horses have an official birthday of January 1 in the Northern Hemisphere, regardless of their actual birth date. This means that all horses in the Kentucky Derby are considered to be three years old as of January 1 of the race year. However, there can be differences in actual maturity since some horses may have been born earlier or later in the year, giving some a developmental advantage
. This three-year-old age rule has been a long-standing tradition in the Kentucky Derby and the entire Triple Crown series, balancing tradition, biology, and competitive fairness
