Taken from the book "The Complete Book of Raising Livestock and Poultry".......
"...when the cockerels are 12 to 14 weeks they are separated from the pullets and confined in a small pen.If left to run loose they will not fatten effectively. I feed them 3 times a day with a mixture of oats,barley and boiled potatoes in equal proportions,mixed with skimmed milk.Including corn in the diet is also a good idea and gives the carcass a more yellow colour. The cockerels are given as much as they will take at each meal,and have access to fresh water at all times. 3 weeks later they are killed for the freezer......"
So anywhere between 15 and 17 weeks is good,according to Katie Thear,Dr.Alistair Fraser and Phil Drabble.