The males can start younger, Jay. Sometimes a much smaller male will get sort of kicked away by a big female but most manage to get the job done after they bug the females enough LOL. Just keep a check on the male that the females aren't picking on him overly. I generally like to put my brand new male breeders in with a few females that have bred before. They seem less bothered by a new and slightly stupid male rat who hasn't quite figured out what end of a female rat is what and is way over excited about being around females.