Rooster is full-grown, I don't know the age for sure, but I'd be surprised if he's under a year old.
My "hens" are mixed ages, from 6 weeks to over a year. After I saw his aggression I separated the 6-week-olds but the 4-month-olds are still with him. I thought that it was the tiny size of the 6-week-olds that made them his targets, but after I took them out I saw him attack a 4-month-old as well; she fared a lot better than the little ones did!
He was at the bottom of the pecking order in the home he was in before. He's only been with me for a week today. I can't really blame him for not knowing how to socialize!
If I separate him from the pullets and hens, in separate but joined pens (so they can still see and talk to each other through the fence), then put him back in with the others after a week or so, might that improve his behavior?