Thoroughbred racehorses all share the same official birthday to standardize age categories crucial for racing fairness and training schedules. In the UK and many northern hemisphere countries, this universal birthday is set as January 1, a rule established by the Jockey Club in 1834 to address inconsistencies caused by poorly kept birth records and to ensure fair competition among horses that begin racing as early as two years old. This system also simplifies breeding and training management, especially for elite breeders like the Royal Stud.

However, the official birthday varies globally to align with regional breeding seasons. For example, in the southern hemisphere, including Australia, the shared birthday is August 1, reflecting the local foaling period from August to December. Despite actual birthdates differing by several months, all thoroughbreds born within the same year are categorized as the same age to maintain consistency in racing eligibility and development comparisons.