With breeds like Australorp, I have butchered at 12 weeks but there is really not much meat there. I don't think it is enough meat to make it worth it, but if you have a reason, like you are not supposed to have roosters, you will get some meat.. I usually wait until at least 15 weeks at the earliest and really prefer 18 weeks.
My situation is probably different than yours. I raise dual purpose chickens for meat as well as eggs and let mine free range. It does not cost me much to feed them after they get out of the brooder. I leave mine on the claw, so to speak, and only butcher a few at a time. That way, if I lose electricirty, I don't have a lot of meat that spoils. It is not unusual for some of mine to be 30 weeks or older before they are butchered, but I usually start around 15 to 18 weeks.