Some FAQs concerning malo can be found here:
http://chins-n-hedgies.com/forums/showthread.php?t=859
Also,
http://www.chinchillas.com/resource/malocclusion.htm
Pretty decent read on an easy-to-access site. As they state, there are many combinations of genetic factors which can cause a chinchilla to malocclude. Any number of chinchillas in any stock, whether hobbyist or rancher, might have animals that when put together for breeding, form a combination of genes in a kit that cause it to malocclude. Hence why it is not so simple as "culling it out". If you had to cull out every trait of an animal that could be a cause of malo, you might not have much of a chinchilla's body remaining.
I think the emphasis on old age was said earlier, that when a chinchilla is older it does not place as much emphasis on chewing as was done at an earlier age. This might not have to be something that is necessarily "documented", as much as it might just be something that is "observed". Also, it seems that a majority of breeders on this site have experience with having malo appearing in their younger animals, not in their older ones. Again, another observation, but maybe not something every young hobby breeder feels they must go out and document in a scientific article or paper.
Besides, how practical is it to even really care what gene combinations cause malo? Like every rancher or breeder has the money to have each of their animal's DNA tested and compared to see if their breeding combinations could cause malo. I would think the best case is luck of the draw. The chance of malo is low, but it could happen. If it does, you do what is necessary to deal with the issue, whether that is euthanasia or filing its teeth down if the owner feels it is in their financial interests to do so. And honestly, I would think it is common sense not to breed the animal suffering from malo after the fact. It could potentially have the full combination of genes to cause the issue, so don't take a chance on letting those genes get passed on to the next poor baby!
And to clarify the topic so it gets back on point, the original questions asked were in concern to breeding age and if there is a definitive and exact way to know whether or not your breeding pair will produce kits with zero problems. As for breeding age, I think it can be any time after they reach a size that makes you comfortable. For me personally, I'm gonna guess 6-7 months, as that is when they tend to hit around 600g. I am sure it differs with others, and I am sure people will share those numbers in this topic. And honestly, we do not breed until after we show, which to this day has yet to be younger than 1 year old. Just something we practice being that we are small and that I do not depend on their breeding as a way to make a living.
As for assuring you that a kit will be perfect from birth until death, I think you know that answer already. We all get old, and we all end up with some sort of problem or another, humans too. I've had an animal I've bought from one of the best who's came home and died within 3 months for unknown reasons (vet post-death exam was inconclusive). I've also had a kit born now that has extremely slow growth, and again, the two animals paired were from top herds and whose prior kits were great animals with no issues whatsoever that was observed. What can you do? I'm definitely not going to pay to have the parents' DNA examined. I seriously doubt they would be found to be "unhealthy", and unworthy of a health certificate. Bad luck in the gene lottery. They won't be bred together again, but who's to say it would happen again? Only way to guarantee that would be to grow them in a petree dish and combine the DNA yourself.
As Therese has said to me, we've domesticated dogs and cats for thousands of years. We've domesticated chinchillas for about 100. There's just too much we still don't know about them, and there's just too many changes still occuring in the species to dare say we have found THE chinchilla whose genetics and health through the span of their lifetime could be labeled as "perfect". If he/she is found however, feel free to PM me as I will quickly respond with my personal mailing address. :thumbsup: