I butcher all my rabbits the day they turn 11 weeks old. No matter the weight or breed of the rabbit. usually they weigh 4 1/2 to 5 pounds. I have a runt new zealand white I am going to butcher today that weighs only 3 1/2 pounds, but is 11 weeks old. My mini lop crosses usually weigh 4 pounds at 11 weeks.
I don't think they'll put on much more weight so in the freezer they go.
squashnut & bassketcher
Champagne D Argent, White New Zealand & Californian Cross Rabbits
I start anytime once they reach 3 1/2 lbs regardless of age but they are usually around 3 to 3 1/2 months. Sometimes I will let them hang around longer if I want a bigger rabbit or watching someone for a possible keeper.
The best age would be less than 12 weeks of age if you want nice, tender fryers. The hormones tend to kick in around 12 weeks of age and that will toughen the meat. When producing for your own table, the size can be whatever you want it to be, but if it's for the meat industry, you'll want 5 lb. rabbits (live weight). It's actually the age that matters most.