Do you have chickens already? If so what kind are they?
There are 3 categories of chickens. Meat types, egg types and dual purpose. Meat types include Cornish X (the kind you find in the grocery store), and slower growing red and black broilers. They produce lots of meat very quickly, and are poor egg layers. Cornish X are usually butchered around 8 weeks of age, the slower growing ones around 12 weeks. Egg types include Leghorns and the hybrid egg layers (red/black sex links, California Greys, etc). They produce a high volume of eggs, and have very little meat. Dual purpose includes everything in between. Rocks, Orpingtons, Austrolorps, etc. They produce a good amount of eggs and still have a reasonable amount of meat. Many butcher the spent hens around 2-3 years of age. It is certainly possible to eat older hens.
What kind of chicken you get will depend on when the best butchering age will be. If you get meat birds then you would probably want to butcher long before you would get any eggs. Get heavy duty egg layers if you want lots of eggs and aren't interested in a meaty carcass. A dual purpose type will be a middle of the road decision.
An important note is that anything other than a meat bird will be much different in taste, texture and size than the grocery store birds. They will require long, slow cooking methods.So if you want the beefy, tender type stick to Cornish X or one of the alternatives, and have a separate layer flock.