I think the question should be how long different varieties maintain their quality as they progress in age in the right water conditions.
For me there are very few hobbyist who keep fish in their pond for more than 5 to 7 years. To answer the question. I have 2 fish which are the oldest in my pond, one a Doitsu Platinum Ogon (10years old) and a Asagi (10 years old). The oldest GoSanke is a Kohaku from Hirasawa (8 years old). I haven't really seen a koi died because of old age, but I have seen how the quality is lose after a certain amount of years.
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IMA NI KOI BITO DE:
The sheep gather and convince themselves that the truth is not the truth.
from the mouth of "Hamburger Helper"...