Hi Barny,

Don't listen to me about horse ages BUT surely it would be the same as any other thing that has a cut off date (school entry, under age sports, etc).

I think they could easily have what you say Barny but I have no idea if horses aren't born all year round.

But, for the sake of getting you an actual answer I thought I'd look it up on Yahoo answers. I don't know if this is correct but this was voted the best answer:

"All thoroughbreds have the same birthday so that their ages can be standardized for comparison because of the historical lack of records of actual birth days. Their birthdays are January 1st in the northern hemisphere and August 1st in the southern hemisphere. After the first time a horse is alive on August 1st it is considered a yearling. The next August 1st is it's 2nd birthday and it is considered 2 years old, even though it could conceivably be one year one day old".