Every single Kentucky Derby horse is (technically) the same age

Each thoroughbred is, technically the same age when they run the Kentucky Derby. But there can be a pretty notable difference in the maturity among the starters when they do.

Officially, the Kentucky Derby is an age-restricted race, meaning only horses of a certain age can enter. For this race (and the entire Triple Crown really) that age is three. Like many of the human sports that require high level athleticism, racehorses tend to front load their lives. Pretty much all horses start racing before they have physically matured, and many of them retire before that as well. Most of the horses in the Derby field began their racing careers as two-year-olds, and every winner in the race’s history has done that.

Officially, every horse on the track May 5 will be exactly the same age. In order to simplify the age requirements, the Jockey Club rules state that every horse’s official birthday is listed as January 1 of the year they were born. This year’s Derby will be made up of horses born in 2015.

That means horses born closer to January 1, have an advantage against their so-called peers at such a young age. To put that another way, in the Derby you could have 2 horses, one born on January 2, and one born on June 2. Both are officially exactly the same age, and eligible for the same races. However one has almost 6 months more of just natural development and growth. So, physically, that one has a slight advantage. And yes, this means that some of the horses running in the Derby likely have not reached their third birthdays yet.

This rule also influences the thoroughbred breeding season. If breeders want their horses in the Derby (which they probably do), the older they are the better. That means there is a fair amount of calculation and possibly some deception that goes into thoroughbred breeding.

The average gestation period for a horse is 342 days, so you can bet the timing of breeding mares is calculated almost to the day. However these models aren’t always accurate. There have been cases (or suspected cases) of thoroughbred farms hiding newborn foals in the last week of December.  This is so they aren’t stuck in an age group with horses nearly a year older. When the horses are this young, having a few more months of development can make a big difference.

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