Every Australian thoroughbred racehorse shares one official birthday, 1 August. AR 161 of the Australian Rules of Racing, made by Racing Australia, reckons a horse's age from 1 August rather than from the day it was foaled, so a whole crop turns a year older together.
So a horse foaled in October 2024 became a two-year-old on 1 August 2026, two to three months before its real second birthday.
How AR 161 sets a horse's age
| Foaled | Dam first covered (Australian Stud Book) | Age counts from |
|---|---|---|
| 1 July to 31 December | On or after 1 September of the previous year | 1 August of the year it was foaled |
| 1 July to 31 December | Before 1 September of the previous year | 1 August of the year before it was foaled |
| 1 January to 30 June | Any date | 1 August of the year before it was foaled |
With illustrative dates, the second row works like this. A mare first covered on 20 August 2024 who foals on 25 July 2025, 339 days later, has a foal whose age counts from 1 August 2024. It turns one on 1 August 2025, seven days after it was born, and is a two-year-old at a year and seven days old.
Why racing gives every horse one birthday
Article 11 of the International Federation of Horseracing Authorities' agreement lets racing authorities count age from a notional date on which every age rises by one, so a crop stays in one age group all season. Australia's date comes just before foaling: AR 161 treats 1 September as the start of the breeding season, and after a pregnancy of about 11 months the first foals arrive in August.
What it means for two- and three-year-olds
Under AR 76 a yearling may not race or run in an official trial. Under AR 77 a two-year-old may not race before 1 October, or another date the state's racing body sets. The limits also bar races over 2,000 metres and, before 1 January (or a date the state sets), handicaps open to older horses. At weight for age a new three-year-old starts light and climbs: over 1400m to 1600m, AR 168 has a three-year-old colt on 50kg in August and 58kg by July.
The shared birthday hides real gaps within a crop. At the Golden Slipper on 21 March 2026, a two-year-old foaled on 5 September 2023 (illustrative dates) was 928 days old and one foaled on 28 December 2023 was 814. Both carried the same set weight for their sex despite a 114-day gap (Golden Slipper betting). Age-restricted races sit across the ladder in Australian race classes.
Southern and northern hemisphere horses
Article 11 also notes that most northern hemisphere authorities use 1 January, and most southern hemisphere ones 1 July or 1 August. A northern hemisphere foal born between January and June counts here from the 1 August before it was born. For an illustrative horse foaled on 15 March 2023:
| Period | Official age in Australia | Age on a 1 January birthday |
|---|---|---|
| 1 August to 31 December 2025 | 3 | 2 |
| 1 January to 31 July 2026 | 3 | 3 |
| 1 August to 31 December 2026 | 4 | 3 |
It shares an official age with a horse foaled here on 10 October 2022 while being 156 days younger. AR 170 offsets that with an allowance in standard weight-for-age races, set out in weight for age.
Where B337 fits
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