I do sell most of my girls when I retire them, but age is usually not a factor to me. I will retire a girl for poor breeding (slow to breed, difficult labor, bad mothering skills), bad lines (if she constantly produces chinchillas that are not of good show standard), or if she doesn't bounce back quickly after deliveries. Some girls are just better suited to be breeders and some aren't. If they don't seem to enjoy the whole Mommy thing I sell them as pets. With the number of chinchillas I have (currently 100) I can't keep every chinchilla I retire and they would get more 1 on 1 attention in a new home.
Right now I am in the process of cutting my herd in half so I am also looking at who produces less kits a year and who is of lesser quality. Those will also go.
I do have a few favorites that I will never part with, though.
As to some breeders selling at a certain age, a lot probably depends on the growth factor. Most breeders like to keep the best chinchillas born to their herd each year to put back into breeding. In my chinchilla building I have more room than I can use so as I add new chinchillas I don't have to retire old breeders. However if I were filled to capacity I would have to retire someone to add someone new. That may be where the 6 year old rule comes in. As a good breeder you always want to be adding new blood, reevaluating your pairs, and culling anyone that isn't measuring up. (cull can be selling or just removing from breeding) Most breeders will reevaluate their herd once a year. This is how you keep producing the best animals you possibly can.