Ross Cobbs aren't hybrids are they? Anyways, I worked on an organic farm that reared them and they went off at 14 weeks, having been free range on grass with ad lib milled oats and grower pellets. Sorry I can't remember how old they were when they came off crumbs and onto pellets, but I think it may have been 4 weeks.
They were really lovely friendly big strong birds. Didn't do much, very laid back, but I liked them.