When do Koi start breeding?

Koi will start breeding when they reach sexual maturity.  This happens when they are at least two years old and about 12 inches long.  If you plan on breeding your Koi, it is better to wait until they are around 4 or 5 years old.  Spawning is stressful and it can damage your Koi.  When they are older, they will be able to handle the stress better.