Horses must be three years old to race in the Kentucky Derby. The race is specifically restricted to three-year-old Thoroughbreds, making it a one-time opportunity for each horse in its lifetime to compete in this event

. The official age of all Thoroughbred racehorses is considered to be the same on January 1 of the year they were born, regardless of their actual birth date. This means that all horses competing in a given Kentucky Derby are technically the same age by this standard, even though some may be physically younger or older by several months

. Most horses begin their racing careers as two-year-olds, and all Kentucky Derby winners have started racing by that age. The race takes place on the first Saturday in May, so the horses are all three years old by that date

. In summary:

• Only three-year-old Thoroughbreds are eligible to race in the Kentucky Derby.
• The official birthday for all Thoroughbreds is January 1 of their birth year.
• Horses typically start racing at two years old before competing in the Derby at three years old.
• The Kentucky Derby is held annually on the first Saturday in May