16 weeks is what I was told- we decided our Barred Rocks were still too bony then, and waited until 18.5 weeks. I cooked them in the crock pot, put them in a chicken pot pie, and they were fabulous. Up to 20 weeks, I think most are still going to be ok, as long as your are careful cooking them- after that, sounds like they are only good in stew. The older they get, the tougher they CAN be, so you want to use moist heat when you cook them, or some have recommended brining them before hand. I was surpised by the amount of meat on the BRs actually- they STILL felt bony to me, but they were quite compact and meaty- I was very pleased. 16 weeks might actually have been just fine.
I would say.........between 15 to 20 weeks is a good age to butcher. I have heard they are the most tender at that time due to the fact they aren't running around gaining muscle and making the meat tough. Just my thoughts.