Sometimes reining classes at breed shows are split into "junior horse" and "senior horse" divisions. Depending on the breed, Junior horses are either 3, 4 or 5 years old, and allowed to show in a snaffle or bosal. Senior horses who age out of the junior horse divisions at age six must be shown in a curb. The rules have changed over the years to reduce the stress on young horses. Junior horse divisions at one time were limited to horses that were only 3 and, sometimes, 4 years old. Expansion to age five parallels the standards set by the FEI and in endurance competitions, recognizing that the physical and mental development of most young horses is not considered complete until that time. Further, though many western stock horse breeds are started under saddle at the age of two, they generally are not physically or mentally ready to be entered into any type of reining competition at that age and in some cases are prohibited from entering any performance class until at least years old.