Smarty Jones will vie for horse racing’s Triple Crown this Saturday at the Belmont Stakes. Like the Crown’s other two jewels, the Kentucky Derby and the Preakness, the Belmont is contested solely by 3-year-old horses. Why is there such a strict age limit, especially considering that older thoroughbreds often run faster?

The predominance of 3-year-olds dates back to the early days of organized English racing. Famous races such as the St. Leger Stakes (first run in 1776), the Epsom Derby (1780), and the Two Thousand Guineas Stakes (1809), which make up the Old World’s version of the Triple Crown, have always been limited to 3-year-old entrants. When the comparable American races started up (beginning with the Belmont in 1867), they were closely patterned on their English predecessors—the Kentucky Derby, for example, took its name from the Epsom Derby, which in turn took its name from an English aristocrat, Lord Derby. In keeping with tradition, then, the American races adopted the age restriction, too.

No one’s entirely sure why the restriction was originally put in place, but it probably has a lot to do with the fact that thoroughbreds generally reach physical maturity during their fourth * year. A race with 2-year-olds would be the equine equivalent of watching a junior-varsity basketball game; a race consisting of 4-year-olds would be something of a snooze, because by that time it would be clear which horses were great and which were lackluster. On the other hand, 3-year olds guarantee both viewing and wagering excitement—they’re old enough to burn up the track, but raw enough to add the element of uncertainty that makes gambling worthwhile.

The English horse owners who first established these races may have also had selfish reasons for instituting the limit. Some racing horses get worn down after they turn 3 * and never race as 4-year-olds; the English horsey set wanted to ensure that their prize thoroughbreds got a chance on the track.

The horses that do soldier onward, however, tend to run faster than their younger counterparts, at least early in the racing season, when the 3-year-olds are still maturing and gaining their competitive sea legs. The early fall marks the first time that 3-year-olds compete against 4-year-olds, most notably in the prestigious Breeders’ Cup Classic. To level the playing field, the Classic’s rules mandates that horses of differing ages be assigned different handicapping weights—3-year-olds are saddled with 122 pounds, while 4-year-olds are weighted down with 126 pounds. Still, 3-year-olds have won fewer than half the Breeders’ Cup Classics that have ever been run.

Next question?

Explainer thanks Ron Mitchell and Steven Haskin of The Blood-Horsemagazine.

Correction, May 24: This piece originally referred to 3-year-old horses as “in their third year.” They are in their fourth year. (Return to the corrected sentence.)