The age and type of race a thoroughbred is eligible to race in is dependent on a number of factors, these are outlined in more detail in the Rules of Racing.  In brief, a horse bred to race on the flat may not race until the start of the flat season in the year of its second birthday, whereas horses who are bred to race over jumps can’t race, at the earliest until 1^st May in the year of its third birthday. There are certain age restrictions on the type of jumps race a horse is eligible to start in therefore many horses will be older than 3yrs old when they race for the first time.