I go by production drop, also; unless, as has already been mentioned, there's another factor.
I've got experience with my breeds, so I know already which ones don't lay well in the cold weather etc.
My 5 year olds are currently giving me 2-3 eggs per day, per 3 hens. But I know that they will eventually begin to drop off, so I'm bringing in chicks this year, so that by spring they will take over and the older girls can lay for as long as they like, or not. When mine stop laying as well, they are just retired, and live out the last of their lives in the same way they lived the first part.
Do you have older hens?
If so: Have they always been with you, and in the same coop? Have you brought in any new birds, or other animals? Have there been any break-in's of their area by predators? Are they slowing down? How old are they, and what are their breeds?