The horses that compete in the Kentucky Derby are all strictly 3 years old. This age requirement applies to all horses in the Triple Crown races as well, meaning a Thoroughbred is eligible for the Kentucky Derby only once in its lifetime
. Officially, every horse’s birthday is considered January 1 of the year they were born to standardize age for racing purposes, so the Derby field consists of horses born in the same calendar year, though some may be physically younger or older depending on their actual foaling date
