How old can the horse be in the Kentucky Derby?
The horse has to be three years old.All horses are considered to have a "birthday" on January 1st, regardless of what was the actual date of birth of each horse. This can be a serious matter in the Kentucky Derby, where only "three year old" horses are eligible to run. If a horse was born on Dec. 31st, it is considered one year older than a horse born on Jan. 1st, and would therefore only be eligible to race in the Derby when it was chronologically less than two and a half years old.