Something to consider, too, is what you want to breed into your flock. (Are you breeding...?)
For me, I would much rather keep an old hen who has slowed down in her laying but who is friendly to her humans and gets along with the reat of the flock without fussiness. I'd love to continue to hatch her eggs, even if they are fewer than when she was young. Or she may have been a survivor of an infection that killed some of your other layers; perhaps she has some inborn immunity.
Just the fact that she has remained healthy for so long counts for something. When you cull out hens as soon as they reach a few years old because you're afraid they may slow down, you never get to see if they actually do. You can choose to breed hens that have a greater long term production if you are patient. Chickens can continue to lay (at reduced rates) for a very, very long time.
I agree with Silkie Chicken: treat each as an individual, but also think of what she can bring to your flock (and your life) in the future.