Chickens in small flocks live several years if conditions are correct. A life span of 6-8 years may not be unreasonable. Most commercial layers are kept for 2-3 years.
They can and do, but it does slow down and can almost stop. I believe bantymum's hen laid one at 16yo.
Most mine rotate though at 4 years or so, but I've kept a favorite for 7 before a coon got it. Into the teens is not unusual for a well kept bird that has had a luckly lease on life and isn't eaten by human or preds.
Seems like everything happens in 20's with chickens. Hatch at 20 - 21 days, lay at 20 weeks, and are good layers til 20 months. Interesting. Maybe start laying golden eggs if they make to 20 years?!?!?!