Hi and welcome to BYC, glad you joined. As others already said, 4 months is about the earliest that cockerels become fertile, but sometimes they need to be several months older, depending on breed and size. And to clarify your question, you are asking about fertility, not how old to "mate." The reason I mention this is because I saw a 9-week-old Easter Egger cockerel "mate" his sister yesterday while she was taking a dust bath. The instinct was there, but not yet the fertility. (I separated him as soon as I saw that.) My point being that dependiing on their age, just because your cockerels begin to mate with your pullets doesn't mean you will immediately get fertile eggs.
If you would like to post a picture of your Orpington, we can confirm if he is Lavender in color.