This is a very interesting question. A horse's ACTUAL age is his age from the day he was born. HOWEVER, for Thoroughbred RACING purposes, all Thoroughbreds become one year older on January 1 of each year, regardless of when they were actually born.
So a horse born on February 6, 2007, and a horse born on April 30, 2007 BOTH become ONE YEAR OLD on January 1, 2008. On January 1 and thereafter they will be the same age for racing purposes, even though they were born almost 3 months apart.
I hope that answers your question.
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By subtracting the year it was born from the current year.
There is no way to "calculate" a horses age. However, if you want to approximate a horses age, then checking their teeth will give you a decent idea - there is a line that appears as a horse ages, and then begins to disappear later in life. While it cannot give you a very accurate (to the year) age, it's length can give you an idea of how old the horse is.
Most American horse registries calculate a horse's age by calander year. Reguardless of the actual date of birth, the horse turns a new age every January. So a foal born in September will be considered a yearling although his actual age would be 3 months.