It really depends on what breeds of sheep they're, the friesland tups are ready in about 5 months. I have one born Jan and he is trying to tup the ewes now, so nearly 5 months, also depends how large they are. The frieslands are early developers, not sure about a lot of other breeds. If I can recollect Welsh are like the Lleyns and don't tend to work until about 8-10 months, they start showing interest at about 3-4 months, but that is just play and they can't work until about 8-10 months, depending on size too. All of these are factors, growth, weight and age. I have noticed sometimes that if their weight isn't as good as it could be they don't show a huge desire. I could be wrong of course, this is just my experience.