I have one hen that will be 2 in March, and she hasn't laid an egg in 4 months. She used to lay 6 eggs a week, sometimes 7... she'd miss maybe 3-4 days a month! I guess she's all laid out. I miss her pretty green eggs.
I have several hens who are 4-6 years old. They are taking longer and longer breaks with each molt, but some of mine are still laying. One bantam Cochin started back laying yesterday after a 7 month break-she'll be 4 in April. By the age of 5, they seem to lay only half the year, maybe slightly more, as long as they aren't laying internally.
I never realized that 2 to 4 year old chickens are considered retirement age. My chickens always lay eggs although not every day; They share nests so I don't ever know which ones are laying. I figured the cold or extreme heat or even drastic temp changes was what made a difference in the number of eggs I was getting. Whatd'ya know...always learning here on BYC!!