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I think this is one of those topics where the precise question asked is very important. Absent the use of hormones, I cannot imagine how rapid growth occurring as a result of perfect conditions and optimal food could possibly shorten the life span of a koi. However, change it to "more than optimal" food, and the development of fatty tissues etc could certainly shorten life spans.
A separate question is the effect on life span of warm climates where the koi never experiences a winter resting period. I do not believe there has been enough study (nor enough experience) to have reliable conclusions. Warm climate koikeeping is just not part of the English language literature on koi.
A further question is determining the "normal" life span of a koi. The stories of 100 year old koi have been de-bunked. But, the notion of carp living over 30 years is not odd. The further one departs from carp to Nishikigoi genes, the less likely such long lives will be lived. Elsewhere JR has posited a life span considerably less than 20 years for show koi of the highest level. Is this due to the in-breeding? Over-feeding? Warm indoor ponds in winter? .... All of these factors and more?
And, how do we decide if a koi died from "old age" or from the innumerable unkindnesses inflicted by the typical koikeeper? And, to what extent do the pristine pond conditions of the best koikeepers extend life well beyond the natural norm? In nature, most die after very short lives. That is why hundreds of thousands of eggs are released with each spawning. A female living 10 years may have laid several million eggs just to produce a replacement pair of adult carp capable of reproducing. At that point, her time has passed, and further egglaying only means other carp were even less successful in maintaining the species.
Well, so far I'm not in fear of any of my koi reaching 20 years! The matriarch enters her 11th summer this year. Hatched from an egg in my little lilypond, she has endured my mistakes and the abuses of climate and disasters. If only the good ones were so hardy and lucky.
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