I have a flock of about 200 large cochins (several varieties) with two 9 year old hens, 6 eight year old hens and another 12 or so over 7 years old and all are laying. When I originally got into large cochins years ago they didn't have very long life spans (3 years max) but now I have, with good management and selection, increased that by several years. They have long productive lives and I have shipped them all over the U.S. for others to enjoy.
Always seek birds from breeders who take seriously point number one in the Historical Introduction of the American Poultry Association's Standard of Perfection which states, "1. That in each breed, the most useful type should be made Standard type."
Birds that don't breed naturally, lay well or represent their standard description should be avoided. Many breeders are guilty of bigger or smaller is better thinking but if you hold to the standard you'll have the proper size, type and quality which leads to healthier, more vigorous birds that in turn will be longer lived.
Bo