Answers are going to differ, even mine from myself.
Variation is in part due to genetics. Currently, my stock represents almost exclusively one strain but we used to make battle crosses using other strains.
Core strain that we developed is where emphasis will be in my answer. About 5-weeks post-hatch is the earliest while the latest is almost 9 months. The range can be realized by offspring of the exact same matings. The early part of range which I assume is causing you grief occurs when I have a fast growing brood and the cock is either not present or especially if he is suddenly removed. Cock somehow suppresses fighting of chicks. The older end of spectrum can be realized also when cock present but with few or no hens on a walk. The cock seems to be the common denominator in he suppresses start of fights. Once fights start with older stags cock cannot stop them and as a rule gets embroiled himself.
The battles on younger side appear to be restricted to siblings of the same clutch. It is usually possible to ID broods preparing to explode the morning before it happens as they start making the aggressive "song" to each other. In the cockyard only setting I am in now the first approach I had was adding a cock / bullstag to a confined brood causing battle royal concerns and the aggression shuts down immediately. That approach has problems in that feeds for young birds are not the same as maintenance ration used for adults. A couple outsourced cocks have also proven also not trustworthy with young birds. Last fall Flypen suggested simply adding a stag to the mix and it works. More than half dozen broods since I have added a stag after pulling hen to suppress fighting. Playing around with this make me think the added stag need only be a little bigger than his charges. I was able to use 8-week old stags over 5-week old broods.
The problem is not restricted to stags, pullets can rain down aggravation as well. That seems to come when hen pulled and they will mix it up with male siblings.
Can you explain more on the aggressive song? I am unsure if I have witnessed them exhibiting that or not... I did have a 3 week old that I had to separate due to fighting though, first time I had ever seen issues in that young of cockerels... he ended up going completely aggressive, manfighter, and was culled...
5 months old seems to be the oldest I have been able to keep cockerels together so far, once the dominance expressing starts I have to separate immediately as they do not stop...
I had wondered about letting them brood with the cockbird, I think that might be a good approach to try, thanks...
Without pullets and hens running loose you should be able to get 5-6 months. With females around all bets are off. Could be 3 months or less. If stags are kept in a pen together much less. I had to remove a few 7 week old stags.
It's not just stags though I have had to pen some pullets and hens. They will fight just as bad as the stags/cocks like centra said.
Ha! The hens and pullets are much more of a challenge, I'd say... males I just need to be careful about other males... the females don't differentiate between gender, breed or age from what I have seen... if they have an issue they just want to settle it, period...