A reader asked me to look into the question of why only three year old horses are allowed to race in the Kentucky Derby. I wasn’t able to find a “for sure” answer, but I did find some information that points in the general direction.

The Kentucky Derby is a Graded Stakes Race. This means that a horses owner must pay a stake, or entry fee, in order to enter the race. The entry fees, plus additional sums posted by the race track, are combine to form the prize money for the race. Graded stakes races traditionally involve horses that belong to the same gender, age and class.

Essentially, it appears as if the age restriction is merely one part of the racing industry’s way of creating competitive races. Age is significant determinate of the competitiveness of two horses.

Relative to humans, horses mature quickly. By height and weight, a horse will be 95% physically mature by the age of two. A two year old horse is probably physiologically equivalent to an eighteen year old human. Mature enough physically to withstand proper training. Horses start training as 2-year olds and will generally only race against other two year olds. Once in a great while, there are races for 2-year olds and up. The training and exertion as a two-year old is helpful in developing the horse’s bone structure.

A horse will reach peak maturity somewhere between the ages of 3.5 and 4-years old. They can usually maintain that peak status through about the age of five.

As a result of these physiological differences, thoroughbred racing often employs a concept known as Weight for Age. A horse will carry a set weight depending on the horse’s age, its sex, the race distance and the month of the year. WFA is used to minimize the differences in physical maturity of a racehorse. By way of example, at the Breeder’s Cup Classic, 4-year-olds and up wear 126 pounds and 3-year-olds wear 122 pounds. The principle of WFA was developed in the 1860s in England and has undergone only minor changes since that time.

For further elaboration, look at the careers of two of the most successful horses of the last several decades, Secretariat and Cigar. Secretariat is the more well known of the two because he won the 1973 Triple Crown. Secretariat set still-standing track records in the Kentucky Derby (1:59 2/5) and the Belmont Stakes (2:24). Secretariat was retired to stud as a three-year old.

Contrast that with Cigar. In 1995 and 1996 Cigar became the first American racehorse racing against top-class competition to win 16 races in a row since the Triple Crown winner Citation did it in 1948 and 1950. He is also the all time leading money earner among North American thoroughbreds, with almost $10 million in career earnings. Cigar’s success came later in life however. Cigar did not race as a two year old and only won twice in nine starts at age 3, but failed to win in stakes competition. Cigar’s first win in a stakes race came in his last race as a 4-year old. He then proceeded to win his next 15 races, tying the winning streak of Citation.