Derby is a type of race for horses. The race is named after Edward Smith-Stanley, 12th Earl of Derby. A Derby only allows a certain age of horse to race. Most Derbies in the United States and the United Kingdom require the horse be three years old to race. So if the horse misses the Kentucky Derby when he is three, he will not get another chance to race in it. Derbies in other countries sometimes require the horse to be four years old.