For age of processing, the primaty question is whether you will skin or pluck. If you want to pluck the birds, the windows of opportunity are at 7, 12.5, and 18 weeks of age. What I mean by window is the stages of new feather growth. If you precess between these times you will encounter many immature (or pin) feathers and will increase the processong time by 2 or 3 times.
If you are to skin them it will not matter.
I feel that after 12.5 weeks there is not really much mass increase as you see earlier. Seems there is more development and feathering.
The Grimaud hybrids I raised showed thick blocky breasts and weighed arouns 8 pounds after dressing. I processed them at 18 weeks, since i was out of country for the 12.5 week window.
make sure you save all the neck as it makes phenominal soup stock.