I have four cockerels marked for the pot; these are birds that I don't want to breed from, as either they're too small to breed useful dual purpose birds, or they're a tad too aggressive (one of the cockerels is particularly harsh on his siblings, even females).
The right time to dispatch chickens varies enormously. Purebreds like sussex take a very long time to develop flesh once they've grown all that bone; indeed, by the time light sussex have fleshed up, they can often be unpleasantly tough. By twelve months of age they're of a good table weight and meatiness, but they've had so many extra months of testosterone that putting them in the pot may mean either stewing or currying them.
Crossing purebreds is a good way to introduce hybrid vigour, which (depending on the cross) should shorten processing age. That's why a lot of people cross sussex to Indian game for table birds, of course adding the Indian's superior meatiness to the mix. The usual result is chicks that develop more quickly than either of the parent birds. More on hybrid vigour in a moment.
However in practice, I've found that Indian game are hard to breed from. If you run a purebred Indian rooster with sussex hens, you'll get a reasonable number of eggs, but you may have none fertile. Indian roosters, unless you get a leggy active one, sometimes have trouble treading more active hens. Conversely if you put the sussex rooster in with Indian game hens, you should get lovely fertile eggs, but due to the Indian's low egg output you won't get many of them. So for me, crossing layers to Indian game isn't the best way to produce useful birds.
In fact, I think purebreds are overrated generally, except in the sense that they allow the retention of genes that would be watered down (and out of existence) when crossing. For instance, I would never have been able to crossbreed using Malay game if I hadn't found some purebreds to begin with. But now that I have the Malay genes, do I have to continue using purebred Malays to produce my desired dual purpose birds?
The answer is a resounding 'No!' Hybrid vigour isn't some magical product of crossing purebreds, but is rather the removal of 'inbreeding depression' that results from breeding pure strains. In this sense crossbreds already exhibit vigour. In other words, when you cross two crossbreds, you don't get increases in vigour between generations, because you start from that basis. Since the only improvements come from genes you introduce to the mix, you can focus on traits you want in the crossing, and without hybrid vigour (or the removal of inbreeding depression) to complicate the results, you can make a truer assessment of where your breeding style is heading. There is therefore no reason, when breeding for utility, to stick to purebreds at all, except when you want to introduce known (and predictable) characteristics, or to increase genetic variation to hopefully result in long term survival against disease.
For these reasons I don't plan to keep crossing purebred Malay game to leghorns or other layer types, but rather to start using some of the offspring as breeders in their own right. The offspring aren't perfect meat birds (as the sussex/Indian cross would be), but I can always bring in new blood of a breed that has the characteristics I want later on. Meanwhile I can keep the hens as layers while taking my time choosing the right table birds and breeding from the remaining roosters.
Now back to age of dispatching cockerels (apologies for taking a long route). Since my birds have lost their inbreeding depression and are maturing fairly fast, I don't need to wait until the cockerels are a year old to be fleshy enough to dispatch. The question of age at putting them in the pot comes down to issues like crowing and pin feathers.
Age of crowing varies a lot between breeds, but so far the ISA/Malay crosses aren't voicing at all, despite the meat hybrid in another pen singing his head off. This is good as it means I don't have to be concerned about my neighbours just yet, and the cockerels have extra time to mature and grow flesh.
The second issue is pin feathers. Chicks moult in various stages, and each time they moult they regrow feathers that start out looking a bit like pins. These narrow pointy feather-buds are very hard to remove from a carcass, and are unsightly (not to mention off-putting to eat) if the skin is left on the bird when roasting or for other purposes. Plucking pin feathers is an horrendous chore, as each one has to be removed between the blunt side of a knife and the thumb. In brief, birds dispatched while still growing pin feathers are best skinned rather than plucked when processing for the pot.
If the skin is needed intact, the best way to end up with a useful table bird is to dispatch when the cockerel is between moults, and his pin feathers have fully emerged. In most chickens there are 4 moults between hatch and maturity, though not all are full-body moults. If you time dispatch between these, depending on the amount of flesh and the maturity of the bird, you should get a decent table carcass.
The first chick moult is a full moult, which occurs between 1 and 6 weeks when all the first feathers grow. The remaining moults are partial, occuring at 7-9 weeks, 12-16 weeks, and 20-22 weeks, depending on the maturation rate of each bird (e.g. some birds begin growing feathers later than others).
To avoid pin feathers, timing of dispatch should therefore be between 16 and 20 weeks, or after 22 weeks. It could also be early in the case of early crowers if noise or behaviour is a problem (say, 9-11 weeks). However for many people relying on medicated chick feed, dispatching at 9-11 weeks brings in the question of meat residues. The bag label may say 'nil withholding period' for meat from birds fed a medicated starter, but personally I'd prefer no medications at all. All the more reason, at least for me, in removing medicated starter as early as possible.
Meanwhile I'm planning to process the 4 excess ISA/Malay cockerels at 16 weeks, namely in 3 weeks' time. Of course it's not going to be fun, and not a day goes by that I don't look at each of the marked 4 boys and think, 'That's a really nice bird.' But it's simply not possible to keep every cockerel, and in the end this is the purpose I bred them for. The main thing in the meantime is to give them as good a life as possible, with fresh air, sunshine, grass, unmedicated feed and as little stress as possible.
Naturally if they began crowing at a young age, I'd have to look at processing them after the 7 week moult, or I could just skin them rather than pluck the carcass. But the nitty gritty of doing all that is best left for another post.
