Thanks for reviving the post; very sound observations from a caring chicken-lover. BTW, my black australorp, Jolene, turned 9 years young this month. I have two Dominiques who'll be 5 in July, and an OE who will be 6 in September.
I too had a hen named Jolene! She was a black sex link and as many have stated above didn't make it to her third birthday due to genetics. There is some great info in this post. I have two 5 year old hens from my first flock that I got from the feed store, my second flock only has 1 remaining 2 year old in it and I have 4 month olds in the brooder. Luck when selecting the actual chick/chicken plays a part for sure! I also think that when having healthy happy chickens common sense plays into it a lot... I don't take there security and their predators for granted. I free range them when I am out in the yard right next to them gardening. I check on them 2 times a day when they are secure in the coop and I often sit with them for long periods of time in the evenings and weekends just to be with them and observe. A big part of chicken health in my opinion is prevention. So I check crops regularly, I am always scooping and observing their poop and I do leg inspections for bumblefoot or anything abnormal. I think if you generally care about your flock, take measures for preventative maintenance, feed them a healthy diet with limited treats you will be surprised by how long they can and will live!
Good luck with your flock and enjoy them!