A racehorse is not considered fully mature until he is aged four. In the Northern Hemisphere the official birthday of all racehorses is January 1st, while in the Southern Hemisphere it is July 1st (South America) or August 1st (Australasia).
The weight-for-age scale was devised in England in the 19th century by one of horseracing's greatest administrators, Admiral Rous, and is still used in a similar format in Europe apart from a few slight adjustments. The weight-for-age scales do vary slightly in the different racing jurisdictions throughout the world but, as the globalisation of racing progresses, a panel of international handicappers is working towards standardization.
Reference to the weight-for-age scale will show how many pounds (or kilos) a horse is deemed immature in any month over any distance. For example, by looking at the allowance the three-year-old receives from older horses over 12 furlongs (2,400 metres) in late July we can see that the amount allowed is 13lbs (6 kg). Therefore, in a weight-for-age race over 12 furlongs in late July where four-year-olds and older are set to carry 9st 7lbs (60 kg or 133lbs), the three-year-old has to carry 8st 8lbs (54.5 kg or 120 lbs). Similarly, the three-year-old receives his weight-for-age allowance in a handicap, so when a three-year-old and an older horse are both rated 95, the three-year-old will carry 13lbs less than the older horse in a race in early July over 12 furlongs (2,400 metres).