I think that most heritage broiler birds were expected to be at good processing weight by 18 weeks. But anything from 16 to 20 seems reasonable. With a 5 lb. dressed bird being at the later end of this spectrum. Based on the chart below I would think that the weights aren't achieved until 24+ weeks. The idea with heritage breeds is that the meat is good at almost any age. From Wikipedia Approximate weight Rooster 3.9 kg 8.5 lbs Hen 2.9 kg 6.5 lbs Ellis was apparently shooting for a broiler but ended up with a good dual purpose bird.