I’ve seen high level show jumpers still competing come 22, they may not be Grand Prix but they have often been sold to someone less experienced so they can learn off the horse.
Many of the red ribbons are on a older pony.
Hello Jessica :)
10-15 is usually prime for mental maturity and physical ability for most horses, but it really depends on each horses. Some will reach it sooner, some later.
It depends on the breed, the temperament, the lifestyle, the work, the level of competition,...
Hope it helps :)
Usually between the ages of 9-12