In theory young Does can be ready to breed by the time they are 4 months old. But you will want to wait until they are at least 8 months of age for breeds like the Katjang. However you must realise that when and if you decide to breed at such a young age you should consider that such young does bred are still growing kids themselves and will need very close monitoring of their feed rations. You must also play close attention to their overall condition as they progress along their pregnancy.
Maturation of any Doe will naturally vary by their genetic/breed background. You cannot expect a smaller breed like a Katjang to be bred to a Boer of a similar age of let’s say at 10 months. We usually let our kids grow up and age until at least 12 months before breeding them in their 2nd year. Talk to the breeders and farmers in your area and I can tell you that there will be mostly different answers.
You will have to then use your own best judgement and common sense to make a decision based on the size and maturity of your own does.