8-10 months is usually the earliest, but it's a good arguement for separating jack and jennet foals at weaning. Most jacks can be started with a gentle, experienced jennet around 18 months. He will need to be taught manners, and not to maul his jennets. He might even settle one or two at that age, but he should be limited to 5-6 jennets until he is over age 2. Another point - If you are going to own only 1 jennet, you probably don't need a jack.
Jacks require special handling and special housing.
This usually means you don't keep the jack and jennet together. Mini jacks ARE cute, and most ARE gentle, but they are still stallions, and they run on testosterone. If you want to have just pets, purchase whichever donkeys personality appeal to you - but have jack colts gelded. If you are going to start serious breeding, buy mature jennets, and the best, proven breeding jack you can find. Raise a foal crop, keep the good daughters, and later rotate in a young jack to breed into the herd.