Well I have a mix of popular layer breeds.
They keep growing even after they start laying.
I think they grew until they were boy, 7 or 8 months old, but it slows down considerably after they hit 16-20 weeks.
I have some American Game and are not considered fully mature until they reach 2years old. They don't grow much after their first year, but they are still growing. 6-12months is when pullets can start to lay and stags start crowing and/or topping hens/pullets. They can reproduce fair offspring before they reach 2years old but produce great offspring when they fully mature. And size will range in mature bird from around 3 1/2-6lbs. I don't know if this is true for Old English.