Final answer:
The Kentucky Derby features 3-year-old Thoroughbreds in a traditional rule marking their maturity for competition.
Step-by-step explanation:
The horses that compete in The Kentucky Derby are all 3 years old. The Kentucky Derby, often referred to as "The Run for the Roses" or "The Fastest Two Minutes in Sports", is an annual horse race held in Louisville, Kentucky. It is a Grade I stakes race for three-year-old Thoroughbreds at a distance of one and a quarter miles at Churchill Downs. The age limit is a traditional rule established for this race, which marks the horse's transition from a juvenile to a more mature competitor.