12 weeks, a bit early but not unusually so. Are the pullets being injured? When I see this type of behavior I generally just walk away as long as none are injured, and they usually are not. This is how they set up flock dynamics, he is establishing his dominance early. To me this is normal chicken behavior. The pullets still grow up to be normal hens, they are not permanently traumatized. They will not like him mating them, they do not want to accept his dominance, but they will get over it as they mature.
It can be hard for some to watch. As someone on here said watching chickens go through puberty is not for the faint at heart. It's probably a lot harder on you than it is the pullets.
If you can't stand to watch it, and some cannot, house him in isolation, next to the pullets so they can see each other but across wire so they can't get together. When most of the pullets are laying, let them back together. He will still establish dominance by mating them, either willingly or against their will, but that's just him establishing dominance so he can perform his duties as flock master. They should soon settle down into a nice calm flock.
If a pullet gets hurt, lock him up until they start to lay. By then the pullets should be mature enough to accept him as flock master.
