Why is the Kentucky Derby only for 3-year-old horses?

The 143rd Kentucky Derby will be held Saturday at Churchill Downs in Louisville and will likely again feature a seemingly endless variety of oddly fascinating women’s hats, but only 3-year-old Thoroughbred racehorses.

Why only 3-year-olds? Why doesn’t arguably the biggest horse race of the year allow the fastest horses to compete against each other, regardless […]