that's the problem, koi can grow at quite different rates. Some may never get over 10-12" full grown. Some could grow to almost two feet in under three years. Depends on water, enviroment, genetics, food, temps, etc. I have a five year old that is still less than 12" long and have a two year old that is 17" Now that being said, just because a fish is old enough to breed doesn't mean it will. I've seen fish that were five years old that have never bred.