I did my australorps at 13 weeks, and they had a lot of meat on them. Maybe not as much as the Cornish X do at 7 weeks, but still enough to make a good meal. Much later than that, your feed conversion ratio starts to drop, and the birds aren't as tender.
If I was going to do it again, I may consider waiting until they were older. I did it earlier so I would have tender meat, but it was already somewhat tough. We're cooking them in ways which require a long, slow cook to help tenderize them. Better flavor than you could ever get at the grocery store, though.