Why are horses sometimes raced before they are old enough?

In horse racing, a horse's birthday is officially classed as the 1st of January each year. So, if a horse is born in September 1988, for example, it will be 1 year old in September 1989. But in racing terms it will turn 1 year old in January 1989, when it is not actually a year old just yet. So the horse will always be 9 months younger than it's racing age, and this sometimes means that a horse is raced before it is truly old enough. I hope this explains it clearly, just get back to me if I confused you.