I have a 13 year old, a 12 year old, an 11 year old, and several 8-10 year olds in my mixed flock. Treating birds well helps, I pride myself in the care of my birds, but that is not the long and short of it. I've had birds that only lived to be 1-2 years old as well. Birds bred for high production of either eggs or meat generally have shorter lifespans no matter how well you treat them. And, of course, if your flock free-ranges, then your local predator population might make a difference in how long your birds live. More often than not, though, chickens die of miscellaneous ailments long before they die of old age, at least in my experience.