For me koi shows are based on size class not age. If you have a yonsai that is 7" inches, it will be placed at the time of benching in it size class. When the judges make their rounds they are only concerned about the fish that day, that is why information about the fish or owner is disclosed to the judges.
But for me it doesn't matter how old a fish is. A koi show is a beauty contest, the best fish wins regardless of age or cosmetic issues.