If the farmer keeps the same breed then he is likely to be experienced in judging when that breed is ready for slaughter. Ask him to show you how he judges and what criteria he uses. In some areas of the continent they prefer a lighter lamb so your breed may or may not be able to grow a little larger without running to fat. Once you send them off, visit the butcher and ask him to show you over the carcases before he chops them up - this is what I have done for the past several years and have learned a lot about raising sheep for meat. Then, once you are eating your animals, judge for yourself if they were slaughtered at the right age. It's all a learning curve.