Unfortunately, we sometimes DON'T get asked this question before the fact, and owners end up with unexpected foals from under-age donkey parents.
Equines are relativley developed at birth - in that the baby can walk, and run on it's first day.
By 4 months, they are nearly ready to switch off mother's milk and start eating grain or grass. They should be weaned by no later than 9 months old, but they are still very immature in body. By age 2, the young equine has assumed more adult proportion in body and leg, but their brains are still young, and their training sessions should be kept short. Most horse breeds will not breed mares until they are a full 2years old, and most wait until they are 3. Sure, her body can sustain a pregnancy, sure, she can give birth - but she's a kid herself.