The Layer Curve
There's lots of good, reliable, concrete information out there about backyard chickens, but most of it is tucked away inside cooperative extension offices and it can be hard to track down. Fortunately, there are lots of these very useful farm bulletins online now; it's just a matter of finding them and sharing them. Here's one I found on factors affecting egg production in backyard flocks. I was particularly surprised to learn that if chickens are without food or water for even a few hours, their egg production will drop off.
I was also relieved to see that on average, even a 10 year-old hen can still lay. Her production might only be 20% of what it once was, but even if that's only one or two eggs a week, it's more than enough to earn her keep as far as I'm concerned.