Unfortunately there is no set age. We all have different goals and set-ups and each bird has its own personality. We have our own tolerances and expectations too. What I might see as normal typical behavior may prompt someone else to call out the national guard to restore order.
I raise mine for meat so my goals are different from yours. I also have a lot of room, three different coops or shelters they can sleep in and over 3,000 square feet outside. I don't know what your facilities look like. I hatch between 40 and 45 chicks a year, some years I have several more girls than boys, some years more boys. Some of my hatches are like yours appear to be, a lot of one sex. Some years all my hatches are like that, but over a two year period the total is usually pretty close to 50%.
Most years I can let them all stay together until butcher age, I start butchering at 16 weeks of age and try to finish by 23 weeks. But every three or four years the boys get so rowdy I isolate several of them from the rest of the flock until they reach butcher age. I have the facilities to do that. Interestingly the girls often outnumber the boys when I have to do that. That's part of why I think it is more of an individua personality thing than a pure boy-girl thing.
Some years the boys start showing an interest in the girls by 12 weeks, and that is usually just one or two when it starts. I don't think I've ever seen it go past 15 weeks before at least one boy get amorous.
If you want to let them get older you can set up a bachelor pad, a separate coop and run where the boys can't get to the girls. That may not tell you a lot about how they will act around girls but it may allow then to get older. I suggest you think about how you would separate one or a few if an emergency pops up. That type of emergency can pop up with a flock of all girls. It is good to be prepared.
As others have mentioned choosing which is not always an easy thing to do. I don't always get it right. Their behaviors change when they hit puberty. After they get through puberty their behaviors change, usually (but not always) for the better. Something that makes it much harder is that the more dominant can suppress the behaviors of the less dominant. What you see as a sweet boy behaving himself can totally change when you remove a more dominant male. Something like a fifth grader being supervised and then suddenly not supervised.
Exactly the process I use. You need to know your goals and start thinning them out based on those goals. I once raised 18 cockerels to determine which I wanted as flock master. Getting down to the final three wasn't all that hard. By the time I got down to three any off them would have been a pretty good choice.
Keep an eye on that one. Even girls can do that but it sounds like a pretty rambunctious boy. Mating behavior is not just about sex, it is dominance behavior. The one on bottom is accepting the dominance of the one on top, either willingly or by force. That's what causes most of the drama when they hit puberty, dominance - not trying to fertilize eggs. That broody hen may have been in shock or may not have even noticed. Sometimes chicks like to climb on Mama, just for play. I'm not sure what was going on.
If you hatch chicks half of them will be boys. You need a plan to deal with the boys. That may be rehoming them, often to someone who will eat them. You may be able to find a zoo or some type of bird of prey rescue that would take them as food for their animals. If you want to learn how to process then yourself you may be able to find a mentor to help you. We get a lot of people on the meat bird section of this forum looking to learn how. You can join us there if you wish, either now or later.
Good luck!
