Three is the most important age for a thoroughbred racehorse
Regardless of when in the year they are born, all horses share the same birthday. Thus a foal who is born in April becomes a yearling (one-year-old) the following 1 January.
There are many yearling sales where trainers, owners and bloodstock agents compete to buy the best-bred and best-looking horses.
The auction circuit begins in the US in July, then moves on to France, Ireland and finally England, where the famous Tattersalls sales take place in Newmarket in November.
The current record price for a yearling is $13.1m (£8.2m) paid for Seattle Dancer in 1985.
Also known as juveniles. A horse is not allowed to race before it is two. There are plenty of valuable races for two-year-olds, particularly in the autumn.
This is the most important year for a Flat horse as this is the 'Classic' year. The five English Classic races - the 1,000 and 2,000 Guineas, the Derby and the St Leger - are restricted to three-year-olds.
Any colt or filly who wins a Classic is likely to be much sought after as a stallion or brood mare.
A horse's good form as a two-year-old is no guarantee of success the following season as over the winter some may not develop as had been expected.
Meanwhile others, who had displayed little aptitude as juveniles, may suddenly make spectacular progress and mature into talented three-year-olds.
Four-year-olds and over
The most successful three-year-olds are often retired at the end of that season to maximise their stud value.
However, there are many valuable races that older horses can also compete in and many who did not shine as three-year-olds mature into top-class performers at four or five.
Flat horses tend to have peaked by five although some continue until they are a lot older. The popular Persian Punch is one such example while in 2002, 10-year-old Yavana's Pace became the oldest horse to win a Group One race in Europe.
Flat horses who do not hit the heights are often sent hurdling at four or five.