Our Hebs jump and 'ride on' the backs of their sisters (and brothers) from about 3 months, although they don't do the biz at that age. But then we don't castrate them either, we just separate the males out from the females at 4 months.
Can you separate the males from the females in your gang and keep them in separate fields? With some breeds, the tale is docked in females but left long in males so it's easy to tell the difference.
It's very likely that with animals intended for selling on, the breeder has been sloppy about castrating the males properly. I have heard slaughterhouse staff saying that from some breeders they get a lot of pregnant ewe hoggs coming through, because they have been run with poorly castrated 'wether' hoggs. The workers are obviously upset when that happens and they find they have killed a pregnant ewe.