I had a cockerel that was raised from about two days old with five pullets.
That little guy was mounting his "roommates" at about 12 weeks old -- long before they were ready for sexual activity. It got so bad that one day, I pulled him out of the henhouse and made him spend the day alone -- not as a punishment, but so that the five little pullets could have at least one day without being attacked by their roo brother.
It was such a relief when the pullets finally reached an age when they could handle his advances....
BTW, this is the same group of babies that ended up producing their own brood before they were six months old.
I'm not kidding -- at five months and about five days old, one of my Buff Orpington pullets decided she wanted to go broody. All we had at the time was the five month old pullets and that one five month old cockerel, so we took some of those little pullet eggs and put them under the little broody.
22 days after we gave that little broody some of her roomie's pullet eggs, she successfully hatched three baby chicks -- one day before these chickens celebrated their six month birthdays!
Those three little baby chicks are now healthy adult chickens, living happily in our backyard...