Traditionally, the term “derby” is used strictly to refer to races restricted to three-year-old thoroughbreds. This age restriction means that a thoroughbred horse is eligible for any given derby only once in their lifetime. The restriction applies to both colts and fillies, though some historical derbies were limited to colts only.

Notable Exceptions
Some modern derbies deviate from the traditional three-year-old restriction. The Hong Kong Derby and Singapore Derby are restricted to four-year-old thoroughbreds, while the Canadian Pacing Derby is for harness racing horses four years old and up.

Historical Origin
The name “derby” itself came about through a coin flip between the 12th Earl of Derby and Sir Charles Bunbury when they were deciding what to name their new race for three-year-old colts. The Earl won the toss, and thus the race became known as the Derby rather than the Bunbury.