You can buy as day olds & sometimes as growers 6 weeks old and off the heat. I buy day old cobb ross hybrids, little monsters they are and as fat as a supermarket rotisserie chicken at 6-7 weeks old.
I buy mine from
http://www.cyril-bason.co.uk/ Day olds were about £1.70 each, growers (5 weeks old) were £3.70 each.
After I have raised them to a good weight I...
Finish on corn
Starve overnight
Kill -broom handle
Drain -blood
Scald -very hot water
Pluck - undress of feathers
Dress -gut, truss up into supermarket shape chicken, wrap in clingfilm.
Rest on plate in fridge for 2-4 days
Freeze or cook.
They eat LOADS, they cost a fortune to feed, they have a nasty habit of 'going off their legs'. When I kept them in a smaller space the meat was tainted, when I put them in the barns with 5 sq meters each they meat was lovely. If you want to free range them get hubbards. Do bear in mind that they will need heat till they are 3-4-5 weeks old depending on how they feather up. I put 12 day old chicks in an 8ft by 4ft brooder + 2 heaters in the barn till they are 2 weeks old, then they get let free into the barn with more heaters till they are 4 weeks old, then the heat gets turned off. If it's winter the heat is on till 5-6 weeks. If they are still on the heat at 5 weeks old you will need more than one heat source as they will pile on-top of each other under the heat and end up suffocating.
None of this has put me off, I usually have some table birds on the go but having a break at the moment so that the barn can recover.