Kentucky Derby horses are all three years old. The race is strictly limited to three-year-old Thoroughbreds, meaning each horse competes in the Derby only once in its lifetime
. Officially, every horse's age is calculated from January 1 of the year they were born, regardless of their actual birthdate, so all horses in a given Derby are considered the same age even if some are physically younger by several months
. This age restriction applies to all three Triple Crown races as well
