You couldn't sell them for much as "meat chickens". They have to be slow cooked because of their age - otherwise the meat is tough. Most people either let them live out a 'pet's life' or cull them (butcher & eat, or sell for less than $4-5).
We cull ours (butcher & eat) before their 3rd year - but ours are not pets. They will lay best up to 2-3 yrs, after that the production drops off. The only stock we keep into the 3rd yr are 1) good breeders - roo or very best layers, 2) good broody/mother.
Here's an example using one of our flocks...
It's a small flock of 8 Barred Rocks that are 19 mo old. They just finished molting and are laying again. They are with a RIR roo. We'll hatch eggs from them for Black Sexlink chicks. They'll turn 2 yrs old at the end of August. We'll move the rooster out and keep them for layers into Fall. The eggs will be sold at the local farmers market along with eggs from our other flocks.
In late October, the market will end. We'll cull the group of hens at that time and butcher them with the Fall crop of Cornish X meat chickens. There are 2 or 3 in the group I may keep for breeding again next season - two best layers and one broody who also lays well.
Now, this would not do at all for some people. For many people their chickens are pets and they would not dream of eating them. But we're a farm, it's business to us. Although we do like our chickens very much and treat them quite well while they're here.
Edited for typo
