Most everything in the equine world is considered a year older on January 1st, regardless of actual birthdays, this goes for horses, riders, and exhibitors. That's why the like to have TB's born as close to Jan 1st as possible, so they can have the most mature horse possible when they are expected to race as a two year old. This rule works great for those horses that have early birthdays, as they are the more mature ones. It works great for exhibitors with early year birthdays too, because they can stay back a division and be the oldest. The only rule in the equine world that I can think of that differs from this is in eventing. Although it might be an USEF thing, so would cross disciplines..... For that, a rider is considered the age that they will turn at whatever point during that year. So, If you turn 22 in December of 2008, you are considered a 22 year old for ALL of 2008.